Aircraft engine

ABSTRACT

A gas turbine engine for an aircraft has an engine core having a turbine, compressor, and core shaft connecting the turbine and compressor; a fan upstream the engine core, the fan having fan blades; and a gearbox. The gearbox receives an input from a core shaft and outputs drive to a fan to drive the fan at a lower rotational speed than the core shaft. The gearbox is an epicyclic gearbox and has a sun gear, planet gears, ring gear, and planet carrier on which the planet gears are mounted. The gearbox has a gear mesh stiffness between the planet gears and the ring gear and a gear mesh stiffness between the planet gears and the sun gear. The gear mesh stiffness between the planet gears and the ring gear divided by that between the planet gears and the sun gear is in the range from 0.90 to 1.28.

FIELD OF INVENTION

The present disclosure relates to gearboxes for use in aircraft engines,to geared gas turbine engines for use in aircraft, and to methods ofoperating such an aircraft. Aspects of the disclosure relate toepicyclic gearboxes having gear mesh stiffnesses meeting specifiedcriteria, and to propulsors for aircraft, such as gas turbine engines,including such a gearbox.

As used herein, a range “from value X to value Y” “between value X andvalue Y”, or the likes, denotes an inclusive range; including thebounding values of X and Y. As used herein, the term “axial plane”denotes a plane extending along the length of an engine, parallel to andcontaining an axial centreline of the engine, and the term “radialplane” denotes a plane extending perpendicular to the axial centrelineof the engine, so including all radial lines at the axial position ofthe radial plane. Axial planes may also be referred to as longitudinalplanes, as they extend along the length of the engine. A radial distanceor an axial distance is therefore a distance extending in a radialdirection in a radial plane, or extending in an axial direction in anaxial plane, respectively.

BRIEF SUMMARY

According to a first aspect there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;a fan located upstream of the engine core, the fan comprising aplurality of fan blades; and a gearbox arranged to receive an input fromthe core shaft and to output drive to the fan so as to drive the fan ata lower rotational speed than the core shaft. The gearbox is anepicyclic gearbox comprising a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted,the gearbox having an overall gear mesh stiffness. The overall gear meshstiffness of the gearbox is greater than or equal to 1.05×10⁹ N/m, andoptionally may be less than or equal to 8.0×10⁹ N/m.

The inventor appreciated that, even with high-precision manufacturing,small errors in alignment of gears (e.g. on the order of 100 μm) and/orin gear tooth shape (e.g. on the order of 10 μm) may be present. Theinventor discovered that providing some flexibility in the overallgearbox gear mesh, and in particular a gearbox mesh stiffness within thedefined range, allows for adjustment for these misalignments or shapeerrors, so improving load share and evenness of wear of gears. At thesame time, too much flexibility in the gear mesh could lead to a lessreliable and/or less efficient gearbox, for example with excessive geartooth deformation and/or excessive torsional vibrations. The inventordiscovered that maintaining overall gearbox mesh stiffness within thespecified range provides optimum gearbox performance.

The gearbox may be defined as having a gearbox diameter defined as thepitch circle diameter (PCD) of the ring gear. The gearbox diameter maybe in the range from 0.55 m to 1.2 m, and optionally from 0.57 to 1.0 m

Turning to gearbox size, and in particular to ring gear pitch circlediameter (PCD) as a measure of gearbox size, the inventor appreciatedthat, in various embodiments, an optimal PCD may also be selected byconsidering the relationship between improved performance due toimproved use of the lever effect for larger gearbox sizes, and theeffect of increased drag for larger gearbox sizes (diminishing returnson the improved lever effect from the larger size above a certain PCD,and increased size and weight of the larger size). Ring gear materialsmay be selected to ensure that a maximum expected torque density for thePCD size would be well within tolerance limits.

Gearbox mesh stiffness may be proportional to the cube of the gear toothsize (also referred to as gear module). The inventor appreciated thatgear tooth size may be invariant with PCD in some arrangements, orproportional to PCD in different embodiments (e.g. maintaining aconstant number of teeth as PCD increases). The gear tooth size mayscale by the square root of the PCD in still other arrangements. Gearboxmesh stiffness may therefore be independent of PCD, or may varyproportionally to PCD to the power of X, where X may optionally be inthe range from 1 to 4 and optionally from 1.5 to 3.

The overall gear mesh stiffness of the gearbox stay be in the range from1.08×10⁹ to 4.9×10⁹ N/m.

A gear mesh stiffness between the planet gears and the ring gear may bein the range from 1.4×10⁹ to 2.0×10¹⁰ N/m.

A gear mesh stiffness between the planet gears and the sun gear may bein the range from 1.20×10⁹ to 1.60×10¹⁰ N/m.

The fan may have a fan diameter in the range from 240 to 280 cm. In suchembodiments, the overall gear mesh stiffness of the gearbox may be inthe range from 1.05×10⁹ to 3.6×10⁹ N/m.

The fan may have a fan diameter in the range from 330 to 380 cm. In suchembodiments, the overall gear mesh stiffness of the gearbox may be inthe range from 1.2×10⁹ to 4.9×10⁹ N/m.

A torsional stiffness of the planet carrier may be greater than or equalto 1.60×10⁸ Nm/rad.

A ring to sun mesh ratio of:

$\frac{\;\begin{matrix}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}}\mspace{11mu}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{ring}\mspace{14mu}{gear}}\end{matrix}}{\mspace{14mu}\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{sun}\mspace{14mu}{gear}}\end{matrix}}$may be in the range from 0.90 to 1.28, and optionally less than or equalto 1.23.

A carrier to sun mesh ratio of:

$\frac{\begin{matrix}{{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}}\mspace{14mu}} \\{{planet}\mspace{14mu}{carrier}}\end{matrix}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$may be greater than or equal to 0.26, and optionally may be greater thanor equal to 4.5.

A carrier to ring mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{11mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}$may be greater than or equal to 0.2, and optionally greater than orequal to 3.8.

The engine may comprise a fan shaft extending between the gearbox andthe fan, and a gearbox support arranged to mount the gearbox within theengine, the fan shaft, core shaft, the gearbox and the gearbox supporttogether forming a transmission.

The effective linear torsional stiffness of the transmission may begreater than or equal to 1.60×10⁸ N/m.

A gear mesh to transmission stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{transmission}}$may be greater than or equal to 0.34, and optionally may be in the rangefrom 0.34 to 11.

According to a second aspect, there is provided a method of operation ofa gas turbine engine for an aircraft, the engine comprising an enginecore comprising a turbine, a compressor, and a core shaft connecting theturbine to the compressor; a fan located upstream of the engine core,the fan comprising a plurality of fan blades; and a gearbox arranged toreceive an input from the core shaft and to output drive to the fan soas to drive the fan at a lower rotational speed than the core shaft. Thegearbox is an epicyclic gearbox comprising a sun gear, a plurality ofplanet gears, a ring gear, and a planet carrier on which the planetgears are mounted, the gearbox having an overall gear mesh stiffness.The overall gear mesh stiffness of the gearbox is greater than or equalto 1.05×10⁹ N/m, and optionally may be less than or equal to 8.0×10⁹N/m. The method comprises operating the gas turbine engine to providepropulsion under cruise conditions.

The gas turbine engine may have any of the features as described withrespect to the first aspect, for example, the PCD of the gearbox may bein the range from 0.55 m to 1.2 m, and optionally from 0.57 to 1.0 m.

The method may comprise driving the gearbox with an input torque ofgreater than or equal to 10,000 Nm at cruise. The method may comprisedriving the gearbox with an input torque of greater than or equal to28,000 Nm at Maximum Take-Off (MTO).

According to a third aspect, there is provided a propulsor for anaircraft, the propulsor comprising a fan comprising a plurality of fanblades; a gearbox; and a power unit for driving the fan via the gearbox.The gearbox is an epicyclic gearbox arranged to receive an input from acore shaft driven by the power unit and to output drive to the fan so asto drive the fan at a lower rotational speed than the core shaft, andcomprises a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The gearbox has anoverall gear mesh stiffness. The overall gear mesh stiffness of thegearbox is greater than or equal to 1.05×10⁹ N/m and optionally may beless than or equal to 8.0×10⁹ N/m.

The propulsor may have some or all of the features as described for thegas turbine engine of the first aspect, for example, the PCD of thegearbox may be in the range from 0.55 m to 1.2 m, and optionally from0.57 to 1.0 m.

According to a fourth aspect, there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;a fan located upstream of the engine core, the fan comprising aplurality of fan blades; and a gearbox arranged to receive an input fromthe core shaft and to output drive to the fan so as to drive the fan ata lower rotational speed than the core shaft. The gearbox is anepicyclic gearbox comprising a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted,the gearbox having a gear mesh stiffness between the planet gears andthe ring gear and a gear mesh stiffness between the planet gears and thesun gear. A ring to sun mesh ratio of:

$\frac{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$is in the range from 0.90 to 1.28.

The inventor appreciated that, even with high-precision manufacturing,small errors in alignment of gears (e.g. on the order of 100 μm) and/orin gear tooth shape (e.g. on the order of 10 μm) may be present. Theinventor discovered that providing some flexibility in the gear meshesbetween the planet gears and the sun and ring gears may allow foradjustment for these misalignments or shape errors, so improving loadshare and evenness of wear of gears. At the same time, too muchflexibility in the gear meshes could lead to a less reliable and/or lessefficient gearbox, for example with excessive gear tooth deformation.The inventor discovered that maintaining the claimed ratio within thespecified range provides optimum gearbox performance.

In particular, the inventor discovered that having more flexibility inthe ring gear as compared to the sun gear (and therefore generally alower value of the ring to sun mesh ratio than in known engines) may bebeneficial in some embodiments, as the ring gear is an internal gear andmay deform further than the sun gear whilst maintain stresses andgearbox functioning within acceptable bounds.

The ring to sun mesh ratio may be in the range from 0.95 to 1.23.

An overall gear mesh stiffness of the gearbox may be greater than orequal to 1.05×10⁹ N/m. The overall gear mesh stiffness of the gearboxmay be in the range from 1.05×10⁹ to 8.00×10⁹ N/m, and optionally from1.08×10⁹ to 4.9×10⁹ N/m.

The gearbox may be defined as having a gearbox diameter defined as thepitch circle diameter of the ring gear. The gearbox diameter may be inthe range from 0.55 to 1.2 m, and optionally from 0.57 to 1.0 m.

The gear mesh stiffness between the planet gears and the ring gear maybe greater than or equal to 1.4×10⁹ N/m, and optionally in the rangefrom 2.45×10⁹ to 1.05×10¹⁰ N/m.

The gear mesh stiffness between the planet gears and the sun gear may begreater than or equal to 1.20×10⁹ N/m, and optionally in the range from2.0×10⁹ to 9.5×10⁹ N/m.

The fan may have a fan diameter in the range from 240 to 280 cm. In suchembodiments, ring to sun mesh ratio may be in the range from 0.95 to1.28.

The fan may have a fan diameter in the range from 330 to 380 cm. In suchembodiments, the ring to sun mesh ratio may be in the range from 0.90 to1.23.

The gear mesh stiffness between the planet gears and the ring gearmultiplied by the gear mesh stiffness between the planet gears and thesun gear may be greater than or equal to 4.7×10¹⁸ N²m⁻², and optionallyless than 1.5×10¹⁹ N⁻².

A torsional stiffness of the planet carrier may be greater than or equalto 1.60×10⁸ Nm/rad.

A carrier to sun mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$may be greater than or equal to 0.26, and optionally greater than orequal to 4.5.

A carrier to ring mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}$may be greater than or equal to 0.2, and optionally greater than orequal to 3.8.

The engine may comprise a fan shaft extending between the gearbox andthe fan, and a gearbox support arranged to mount the gearbox within theengine, the fan shaft, core shaft, the gearbox, and the gearbox supporttogether forming a transmission.

A gear mesh to transmission stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{transmission}}$may be in the range from 0.34 to 11.

The effective linear torsional stiffness of the transmission may begreater than or equal to 1.60×10⁸ N/m or 3.8×10⁸ N/m.

According to a fifth aspect, there is provided a method of operation ofa gas turbine engine for an aircraft, the engine comprising an enginecore comprising a turbine, a compressor, and a core shaft connecting theturbine to the compressor; a fan located upstream of the engine core,the fan comprising a plurality of fan blades; and a gearbox arranged toreceive an input from the core shaft and to output drive to the fan soas to drive the fan at a lower rotational speed than the core shaft. Thegearbox is an epicyclic gearbox comprising a sun gear, a plurality ofplanet gears, a ring gear, and a planet carrier on which the planetgears are mounted. The gearbox has a gear mesh stiffness between theplanet gears and the ring gear and a gear mesh stiffness between theplanet gears and the sun gear. A ring to sun mesh ratio of:

$\frac{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$is in the range from 0.90 to 1.28. The method comprises operating thegas turbine engine to provide propulsion under cruise conditions.

The gas turbine engine may have any or all of the features as describedwith respect to the fourth aspect.

The method may comprise driving the gearbox with an input torque ofgreater than or equal to 10,000 Nm at cruise. The method may comprisedriving the gearbox with an input torque of greater than or equal to28,000 Nm at Maximum Take-Off (MTO).

According to a sixth aspect, there is provided a propulsor for anaircraft, the propulsor comprising a fan comprising a plurality of fanblades; a gearbox; and a power unit for driving the fan via the gearbox.The gearbox is an epicyclic gearbox arranged to receive an input from acore shaft and to output drive to a fan so as to drive the fan at alower rotational speed than the core shaft, the gearbox comprising a sungear, a plurality of planet gears, a ring gear, and a planet carrier onwhich the planet gears are mounted. The gearbox has a gear meshstiffness between the planet gears and the ring gear and a gear meshstiffness between the planet gears and the sun gear. A ring to sun meshratio of:

$\frac{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$is in the range from 0.90 to 1.28.

The propulsor may have any or all of the features as described for thegas turbine engine of the fourth aspect.

In other aspects, value ranges for product of the components of the ringto sun mesh ratio may be specified instead of, or as well as, valueranges for the ratios.

In particular, ring and sun mesh product defined as the gear meshstiffness between the planet gears and the ring gear multiplied by thegear mesh stiffness between the planet gears and the sun gear may have avalue greater than or equal to 4.7×10¹⁸N²m⁻², and optionally less than1.5×10¹⁹ N²m⁻², and optionally may be greater than or equal to5.1×10¹⁸N²m⁻², and optionally less than 1.3×10¹⁹ N²m⁻².

According to one such aspect, there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;a fan located upstream of the engine core, the fan comprising aplurality of fan blades; and a gearbox arranged to receive an input fromthe core shaft and to output drive to the fan so as to drive the fan ata lower rotational speed than the core shaft. The gearbox is anepicyclic gearbox comprising a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted,the gearbox having a gear mesh stiffness between the planet gears andthe ring gear and a gear mesh stiffness between the planet gears and thesun gear. A ring and sun mesh product defined as the gear mesh stiffnessbetween the planet gears and the ring gear multiplied by the gear meshstiffness between the planet gears and the sun gear is greater than orequal to 4.7×10¹⁸ N²m⁻².

The skilled person would appreciate that method and propulsor aspectsmay be formulated accordingly.

According to a seventh aspect, there is provided a gas turbine enginefor an aircraft, the engine comprising an engine core comprising aturbine, a compressor, and a core shaft connecting the turbine to thecompressor; a fan located upstream of the engine core, the fancomprising a plurality of fan blades; and a gearbox arranged to receivean input from the core shaft and to output drive to the fan so as todrive the fan at a lower rotational speed than the core shaft. Thegearbox is an epicyclic gearbox comprising a sun gear, a plurality ofplanet gears, a ring gear, and a planet carrier on which the planetgears are mounted. The planet carrier has an effective linear torsionalstiffness and the gearbox has a gear mesh stiffness between the planetgears and the sun gear. A carrier to sun mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$is greater than or equal to 0.26, and optionally may be less than orequal to 1.1×10³.

The inventor discovered that the torsional stiffness of the carrier maybeneficially be arranged to be relatively high compared to the gear meshstiffness between the planet gears and the sun gear (as compared toknown engines) so as to reduce or avoid wind-up of the gearbox, soreducing or eliminating bending of teeth, whilst still allowing someflexibility within the gear mesh between the planet gears and the sungear to accommodate differences between teeth and/or gears. The inventordiscovered that maintaining the specified ratio within the claimed rangeprovides optimum gearbox performance. An effective linear torsionalstiffness of the carrier is used for ease of comparison with the(linear) gear mesh stiffness.

The carrier to sun mesh ratio may be greater than or equal to 4.5.

An overall gear mesh stiffness of the gearbox may be greater than orequal to 1.05×10⁹ N/m.

The gearbox may be defined as having a gearbox diameter defined as thepitch circle diameter (PCD) of the ring gear. The gearbox diameter maybe in the range from 0.55 m to 1.2 m, and optionally from 0.57 to 1.0 m.

A gear mesh stiffness between the planet gears and the ring gear may begreater than or equal to 1.4×10⁹ N/m, and optionally in the range from2.45×10⁹ to 1.05×10¹⁰ N/m.

The gear mesh stiffness between the planet gears and the sun gear may begreater than or equal to 1.20×10⁹ N/m, and optionally in the range from2.0×10⁹ to 9.5×10⁹ N/m.

The fan may have a fan diameter in the range from 240 to 280 cm. In suchembodiments, the carrier to sun mesh ratio may be in the range from 0.6to 58.

The fan may have a fan diameter in the range from 330 to 380 cm. In suchembodiments, the carrier to sun mesh ratio may be in the range from 0.94to 95.

The effective linear torsional stiffness of the planet carrier may begreater than or equal to 7.00×10⁹ N/m.

The product of the effective linear torsional stiffness of the planetcarrier and the gear mesh stiffness between the planet gears and the sungear may be greater than or equal to 5.0×10¹⁸ N²m⁻².

A ring to sun mesh ratio of:

$\frac{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$may be in the range from 0.90 to 1.28.

A carrier to ring mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{ring}\mspace{14mu}{gear}}\end{matrix}}$may be greater than or equal to 0.2, and optionally greater than orequal to 3.8.

The gas turbine engine may comprise a fan shaft extending between thegearbox and the fan, and a gearbox support arranged to mount the gearboxwithin the engine, the fan shaft, core shaft, gearbox, and gearboxsupport together forming a transmission.

A gear mesh to transmission stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{transmission}}$may be in the range from 0.34 to 11.

The effective linear torsional stiffness of the transmission may begreater than or equal to 1.60×10⁸ N/m.

According to an eighth aspect, there is provided a method of operationof a gas turbine engine for an aircraft, the engine comprising an enginecore comprising a turbine, a compressor, and a core shaft connecting theturbine to the compressor; a fan located upstream of the engine core,the fan comprising a plurality of fan blades; and a gearbox arranged toreceive an input from the core shaft and to output drive to the fan soas to drive the fan at a lower rotational speed than the core shaft. Thegearbox is an epicyclic gearbox comprising a sun gear, a plurality ofplanet gears, a ring gear, and a planet carrier on which the planetgears are mounted. The planet carrier has an effective linear torsionalstiffness and the gearbox has a gear mesh stiffness between the planetgears and the sun gear. A carrier to sun mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$is greater than or equal to 0.26, and optionally may be less than orequal to 1.1×10³. The method comprises operating the gas turbine engineto provide propulsion under cruise conditions.

The gas turbine engine may have any or all of the features as describedwith respect to the seventh aspect.

The method may comprise driving the gearbox with an input torque ofgreater than or equal to 10,000 Nm at cruise. The method may comprisedriving the gearbox with an input torque of greater than or equal to28,000 Nm at Maximum Take-Off (MTO).

According to a ninth aspect, there is provided a propulsor for anaircraft, the propulsor comprising a fan comprising a plurality of fanblades; a gearbox; and a power unit for driving the fan via the gearbox.The gearbox is an epicyclic gearbox arranged to receive an input from acore shaft driven by the power unit and to output drive to the fan so asto drive the fan at a lower rotational speed than the core shaft, andcomprises a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The planet carrierhas an effective linear torsional stiffness and the gearbox has a gearmesh stiffness between the planet gears and the sun gear. A carrier tosun mesh ratio of:

$\frac{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{11mu}{the}} \\{{sun}\mspace{14mu}{gear}}\end{matrix}}$is greater than or equal to 0.26, and optionally may be less than orequal to 1.1×10³.

The propulsor may have any or all of the features as described for thegas turbine engine of the seventh aspect.

In other aspects, value ranges for a product of the components of thecarrier to sun mesh ratio may be specified instead of, or as well as, avalue range for the ratio.

In particular, a carrier and sun mesh product defined as the effectivelinear torsional stiffness of the planet carrier multiplied by the gearmesh stiffness between the planet gears and the sun gear may have avalue greater than or equal to 5.0×10¹⁸ N²m⁻², and optionally less than2.0×10²² N²m⁻², and optionally may be greater than or equal to1.8×10¹⁹N²m⁻², and optionally less than 1.0×10²¹ N²m⁻².

According to one such aspect, there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;a fan located upstream of the engine core, the fan comprising aplurality of fan blades; and a gearbox arranged to receive an input fromthe core shaft and to output drive to the fan so as to drive the fan ata lower rotational speed than the core shaft. The gearbox is anepicyclic gearbox comprising a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted.The planet carrier has an effective linear torsional stiffness and thegearbox has a gear mesh stiffness between the planet gears and the sungear. A carrier and sun mesh product defined as the effective lineartorsional stiffness of the planet carrier multiplied by the gear meshstiffness between the planet gears and the sun gear is value greaterthan or equal to 5.0×10¹⁸ N²m⁻².

The skilled person would appreciate that method and propulsor aspectsmay be formulated accordingly.

According to a tenth aspect, there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;a fan located upstream of the engine core, the fan comprising aplurality of fan blades; and a gearbox arranged to receive an input fromthe core shaft and to output drive to the fan so as to drive the fan ata lower rotational speed than the core shaft. The gearbox is anepicyclic gearbox comprising a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted.The planet carrier has an effective linear torsional stiffness and thegearbox has a gear mesh stiffness between the planet gears and the ringgear. A carrier to ring mesh ratio of:

$\frac{{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}\;}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{11mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\;{ring}\mspace{14mu}{gear}}\;}$is greater than or equal to 0.2, and optionally may be less than orequal to 900.

The inventor discovered that the torsional stiffness of the carrier maybeneficially be arranged to be relatively high compared to the gear meshstiffness between the planet gears and the sun gear so as to reduce oravoid wind-up of the gearbox, so reducing or eliminating bending ofteeth, whilst still allowing some flexibility within the gear meshbetween the planet gears and the ring gear to accommodate differencesbetween teeth and/or gears. The inventor discovered that maintaining thespecified ratio within the claimed range can provide optimum gearboxperformance. An effective linear torsional stiffness of the carrier isused for ease of comparison with the (linear) gear mesh stiffness.

The carrier to ring mesh ratio may be greater than or equal to 3.8.

An overall gear mesh stiffness of the gearbox may be in the range from1.05×10⁹ to 8.00×10⁹ N/m, and optionally in the range from 1.08×10⁹ to4.9×10⁹ N/m.

The gearbox may be defined as having a gearbox diameter defined as thepitch circle diameter (PCD) of the ring gear. The gearbox diameter maybe in the range from 0.55 m to 1.2 m, and optionally from 0.57 to 1.0 m.

The gear mesh stiffness between the planet gears and the ring gear maybe greater than or equal to 1.4×10⁹ N/m, and optionally in the rangefrom 2.45×10⁹ to 1.05×10¹⁰ N/m.

A gear mesh stiffness between the planet gears and the sun gear may begreater than or equal to 1.20×10⁹ N/m, and optionally in the range from2.0×10⁹ to 9.5×10⁹ N/m.

The fan may have a fan diameter in the range from 240 to 280 cm. In suchembodiments, the carrier to ring mesh ratio may be greater than or equalto 3.8, and optionally in the range from 3.8 to 90.

The fan may have a fan diameter in the range from 330 to 380 cm. In suchembodiments, the carrier to ring mesh ratio may be greater than or equalto 4.0, and optionally in the range from 4.0 to 500.

The effective linear torsional stiffness of the planet carrier may begreater than or equal to 7.00×10⁹ N/m.

The product of the effective linear torsional stiffness of the planetcarrier and the gear mesh stiffness between the planet gears and thering gear may be greater than or equal to 5.0×10¹⁸ N²m⁻².

A ring to sun mesh ratio of:

$\frac{\;\begin{matrix}{{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}}\mspace{11mu}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\;{ring}\mspace{14mu}{gear}}\end{matrix}}{\mspace{14mu}\begin{matrix}{{the}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\;{sun}\mspace{14mu}{gear}}\end{matrix}}$may be in the range from 0.90 to 1.28.

A carrier to sun mesh ratio of:

$\frac{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\;{planet}\mspace{14mu}{carrier}}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{sun}\mspace{14mu}{gear}}\;}$may be greater than or equal to 0.26, and optionally greater than orequal to 4.5.

The gas turbine engine may comprise a fan shaft extending between thegearbox and the fan, and a gearbox support arranged to mount the gearboxwithin the engine, the fan shaft, core shaft, gearbox and gearboxsupport together forming a transmission. A gear mesh to transmissionstiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{transmission}}$may be in the range from 0.34 to 11.

The effective linear torsional stiffness of the transmission may begreater than or equal to 1.60×10⁸ N/m.

According to an eleventh aspect, there is provided a method of operationof a gas turbine engine for an aircraft, the engine comprising an enginecore comprising a turbine, a compressor, and a core shaft connecting theturbine to the compressor; a fan located upstream of the engine core,the fan comprising a plurality of fan blades; and a gearbox arranged toreceive an input from the core shaft and to output drive to the fan soas to drive the fan at a lower rotational speed than the core shaft. Thegearbox is an epicyclic gearbox comprising a sun gear, a plurality ofplanet gears, a ring gear, and a planet carrier on which the planetgears are mounted. The planet carrier has an effective linear torsionalstiffness and the gearbox has a gear mesh stiffness between the planetgears and the ring gear. A carrier to ring mesh ratio of:

$\frac{{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}\;}{{\mspace{11mu}\;\;}\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{ring}\mspace{14mu}{gear}}\end{matrix}}$is greater than or equal to 0.2, and optionally may be less than orequal to 900. The method comprises operating the gas turbine engine toprovide propulsion under cruise conditions.

The gas turbine engine may be as described with respect to the tenthaspect.

The method may comprise driving the gearbox with an input torque ofgreater than or equal to 10,000 Nm at cruise. The method may comprisedriving the gearbox with an input torque of greater than or equal to28,000 Nm at Maximum Take-Off (MTO).

According to a twelfth aspect, there is provided a propulsor for anaircraft, the propulsor comprising a fan comprising a plurality of fanblades; a gearbox; and a power unit for driving the fan via the gearbox.The gearbox is an epicyclic gearbox arranged to receive an input from acore shaft driven by the power unit and to output drive to the fan so asto drive the fan at a lower rotational speed than the core shaft, andcomprises a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The planet carrierhas an effective linear torsional stiffness and the gearbox has a gearmesh stiffness between the planet gears and the ring gear. A carrier toring mesh ratio of:

$\frac{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}{{\mspace{11mu}\;\;}\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{ring}\mspace{14mu}{gear}}\end{matrix}}$is greater than or equal to 0.2, and optionally may be less than orequal to 900.

The propulsor may have any or all of the features as described for thegas turbine engine of the tenth aspect.

In other aspects, value ranges for the product of the components of thecarrier to ring mesh ratio may be specified instead of, or as well as, avalue range for the ratio.

In particular, a carrier and ring mesh product defined as the effectivelinear torsional stiffness of the planet carrier multiplied by the gearmesh stiffness between the planet gears and the ring gear may be greaterthan or equal to 5.0×10¹⁸ N²m⁻², and optionally less than 2.6×10²²N²m⁻², and optionally may be greater than or equal to 2.2×10¹⁹N²m⁻², andoptionally less than 2.6×10²¹ N²m⁻².

According to one such aspect, there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;a fan located upstream of the engine core, the fan comprising aplurality of fan blades; and a gearbox arranged to receive an input fromthe core shaft and to output drive to the fan so as to drive the fan ata lower rotational speed than the core shaft. The gearbox is anepicyclic gearbox comprising a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted.The planet carrier has an effective linear torsional stiffness and thegearbox has a gear mesh stiffness between the planet gears and the ringgear. A carrier and ring mesh product defined as the effective lineartorsional stiffness of the planet carrier multiplied by the gear meshstiffness between the planet gears and the ring gear is greater than orequal to 5.0×10¹⁸ N²m⁻².

The skilled person would appreciate that method and propulsor aspectsmay be formulated accordingly.

According to a thirteenth aspect, there is provided a gas turbine enginefor an aircraft, the engine comprising an engine core comprising aturbine, a compressor, a gearbox, a gearbox support arranged to mountthe gearbox within the engine, and a core shaft connecting the turbineto the compressor; a fan located upstream of the engine core, the fancomprising a plurality of fan blades; and a fan shaft. The gearbox isarranged to receive an input from the core shaft and to output drive tothe fan via the fan shaft so as to drive the fan at a lower rotationalspeed than the core shaft. The gearbox is an epicyclic gearboxcomprising a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The gearbox has anoverall gear mesh stiffness. The fan shaft, the core shaft, the gearbox,and the gearbox support together form a transmission having an effectivelinear torsional stiffness. A gear mesh to transmission stiffness ratioof:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{transmission}}$is greater than or equal to 0.34.

The gear mesh to transmission stiffness ratio may be in the range from0.34 to 11, or from 0.90 to 4.6.

The fan may have a fan diameter in the range from 240 to 280 cm. In suchembodiments, the gear mesh to transmission stiffness ratio may be in therange from 1.4 to 2.7.

The fan may have a fan diameter in the range from 330 to 380 cm. In suchembodiments, the gear mesh to transmission stiffness ratio may be in therange from 0.5 to 4.6.

According to a fourteenth aspect, there is provided a gas turbine enginefor an aircraft, the engine comprising an engine core comprising aturbine, a compressor, a gearbox, a gearbox support arranged to mountthe gearbox within the engine, the gearbox support having an effectivelinear torsional stiffness, and a core shaft connecting the turbine tothe compressor; a fan located upstream of the engine core, the fancomprising a plurality of fan blades; and a fan shaft. The gearbox isarranged to receive an input from the core shaft and to output drive tothe fan via the fan shaft so as to drive the fan at a lower rotationalspeed than the core shaft. The gearbox is an epicyclic gearboxcomprising a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The gearbox has anoverall gear mesh stiffness. A gear mesh to gearbox support stiffnessratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu}{support}}$is greater than or equal to 6.5×10⁻², and optionally less than or equalto 2.6×10¹.

The gear mesh to gearbox support stiffness ratio may be greater than orequal to 2.6×10⁻¹, and optionally in the range from 0.26 to 8.0.

According to a fifteenth aspect, there is provided a gas turbine enginefor an aircraft, the engine comprising an engine core comprising aturbine, a compressor, a gearbox, a gearbox support arranged to mountthe gearbox within the engine, and a core shaft connecting the turbineto the compressor; a fan located upstream of the engine core, the fancomprising a plurality of fan blades; and a fan shaft, the fan shafthaving an effective linear torsional stiffness. The gearbox is arrangedto receive an input from the core shaft and to output drive to the fanvia the fan shaft so as to drive the fan at a lower rotational speedthan the core shaft. The gearbox is an epicyclic gearbox comprising asun gear, a plurality of planet gears, a ring gear, and a planet carrieron which the planet gears are mounted. The gearbox has an overall gearmesh stiffness. A gear mesh to fan shaft stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{fan}\mspace{14mu}{shaft}}$is in the range from 0.3 to 1.6.

According to a sixteenth aspect, there is provided a gas turbine enginefor an aircraft, the engine comprising an engine core comprising aturbine, a compressor, a gearbox, a gearbox support arranged to mountthe gearbox within the engine, and a core shaft connecting the turbineto the compressor, the core shaft having an effective linear torsionalstiffness; a fan located upstream of the engine core, the fan comprisinga plurality of fan blades; and a fan shaft. The gearbox is arranged toreceive an input from the core shaft and to output drive to the fan viathe fan shaft so as to drive the fan at a lower rotational speed thanthe core shaft. The gearbox is an epicyclic gearbox comprising a sungear, a plurality of planet gears, a ring gear, and a planet carrier onwhich the planet gears are mounted. The gearbox has an overall gear meshstiffness. A gear mesh to core shaft stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{core}\mspace{14mu}{shaft}}$is greater than or equal to 0.20. The gear mesh to core shaft stiffnessratio may be in the range from 0.20 to 90, and optionally from 0.20 to29.

The inventor has discovered that the stiffness of a transmissionincluding in particular the overall gear mesh stiffness of the gearboxand the torsional stiffness of the transmission—should be distributed inone or more of the claimed proportions to allow some flexibility withinthe gear mesh between the gears to accommodate differences between teethand/or gears whilst reducing or avoiding excessive deflections due totorsional vibrations. The transmission includes the fan shaft, gearboxinput shaft, and gearbox support. The inventor appreciated that thetorsional stiffnesses of each component may therefore be considered inthe same way, as compared to the overall gear mesh stiffness, anddiscovered that maintaining the corresponding ratios within one or moreof the described ranges may provide optimum gearbox performance. Aneffective linear torsional stiffness of the transmission is used forease of comparison with the (linear) gear mesh stiffness.

The inventor has realised that decreasing the torsional stiffnesses ofthe transmission below the ranges defined herein may result indeleterious torsion vibrations at low modal frequencies (the skilledperson would appreciate that the lower modal frequency whirl modes havelarger amplitudes/deflections than the higher modes, and so are moreimportant to avoid), whilst increasing the torsional stiffness above theranges defined herein may result in excessive size and/or weight of theshaft without a corresponding improvement in performance.

Turning to the core shaft/the gearbox input shaft, the inventor hasdiscovered that the torsional stiffness of the gearbox input shaft hasan effect on the torsional stiffness of the whole transmission, but arelatively minimal effect on gearbox operation as torsional deflectionresults in wind up only, and no misalignment of gears. The gearbox inputshaft may therefore have a lower torsional stiffness than the carrierwithout deleterious effects. Similar considerations may apply to the fanshaft (the gearbox output shaft).

The inventor has realised that decreasing the torsional stiffnesses ofthe shafts below the ranges defined herein may result in deleterioustorsion vibrations at low modal frequencies (the skilled person wouldappreciate that the lower modal frequency whirl modes have largeramplitudes/deflections than the higher modes, and so are more importantto avoid), whilst increasing the torsional stiffness above the rangesdefined herein may result in excessive size and/or weight of the shaftwithout a corresponding improvement in performance.

In any of the thirteenth to sixteenth aspects, one or more of thefollowing may apply:

The overall gear mesh stiffness of the gearbox may be greater than orequal to 1.05×10⁹ N/m.

The gearbox may be defined as having a gearbox diameter defined as thepitch circle diameter (PCD) of the ring gear. The gearbox diameter maybe in the range from 0.55 m to 1.2 m, and optionally from 0.57 to 1.0 m.

A gear mesh stiffness between the planet gears and the ring gear may begreater than or equal to 1.40×10⁹ N/m, and optionally in the range from2.45×10⁹ to 1.05×10¹⁰ N/m.

A gear mesh stiffness between the planet gears and the sun gear may begreater than or equal to 1.20×10⁹ N/m, and optionally in the range from2.0×10⁹ to 9.5×10⁹ N/m.

A ring to sun mesh ratio of:

$\frac{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{11mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{ring}\mspace{14mu}{gear}}\;}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{sun}\mspace{14mu}{gear}}\;}$may be in the range from 0.90 to 1.28.

A carrier to sun mesh ratio of:

$\frac{{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}\;}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{sun}\mspace{14mu}{gear}\;}$may be greater than or equal to 0.26, and optionally greater than orequal to 4.5.

A carrier to ring mesh ratio of:

$\frac{{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{carrier}}\;}{{\mspace{11mu}\mspace{20mu}}\begin{matrix}{\;{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}}} \\{{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{ring}\mspace{14mu}{gear}}\end{matrix}}$may be greater than or equal to 0.2, and optionally greater than orequal to 3.8. The effective linear torsional stiffness of the core shaftmay be greater than or equal to 4.0×10⁸ N/m.

The effective linear torsional stiffness of the fan shaft may be greaterthan or equal to 1.2×10⁹ N/m.

The effective linear torsional stiffness of the gearbox support may begreater than or equal to 7.1×10⁸ N/m.

The effective linear torsional stiffness of the planet carrier may begreater than or equal to 7.0×10⁹ N/m.

According to a seventeenth aspect, there is provided a method ofoperation of a gas turbine engine for an aircraft, the engine comprisingan engine core comprising a turbine, a compressor, and a core shaftconnecting the turbine to the compressor; a fan located upstream of theengine core, the fan comprising a plurality of fan blades; a gearboxarranged to receive an input from the core shaft and to output drive tothe fan so as to drive the fan at a lower rotational speed than the coreshaft, the gearbox being an epicyclic gearbox comprising a sun gear, aplurality of planet gears, a ring gear, and a planet carrier on whichthe planet gears are mounted, and the gearbox having an overall gearmesh stiffness; and a gearbox support arranged to mount the gearboxwithin the engine. The fan shaft, core shaft, gearbox, and gearboxsupport together form a transmission having an effective lineartorsional stiffness. A gear mesh to transmission stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{11mu}{transmission}}$is greater than or equal to 0.34, and optionally may be less than orequal to 11. The method comprises operating the gas turbine engine toprovide propulsion under cruise conditions.

According to an eighteenth aspect, there is provided a method ofoperation of a gas turbine engine for an aircraft, the engine comprisingan engine core comprising a turbine, a compressor, and a core shaftconnecting the turbine to the compressor; a fan located upstream of theengine core, the fan comprising a plurality of fan blades; a gearboxarranged to receive an input from the core shaft and to output drive tothe fan so as to drive the fan at a lower rotational speed than the coreshaft, and a gearbox support arranged to mount the gearbox within theengine, the gearbox support having an effective linear torsionalstiffness. The gearbox is an epicyclic gearbox comprising a sun gear, aplurality of planet gears, a ring gear, and a planet carrier on whichthe planet gears are mounted. The gearbox has an overall gear meshstiffness. A gear mesh to gearbox support stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu}{support}}$is greater than or equal to 6.5×10⁻², and optionally may be less than orequal to 2.6×10¹. The method comprises operating the gas turbine engineto provide propulsion under cruise conditions.

According to a nineteenth aspect, there is provided a method ofoperation of a gas turbine engine for an aircraft, the engine comprisingan engine core comprising a turbine, a compressor, and a core shaftconnecting the turbine to the compressor; a fan located upstream of theengine core, the fan comprising a plurality of fan blades; a fan shaft,the fan shaft having an effective linear torsional stiffness; and agearbox arranged to receive an input from the core shaft and to outputdrive to the fan via the fan shaft so as to drive the fan at a lowerrotational speed than the core shaft. The gearbox is an epicyclicgearbox comprising a sun gear, a plurality of planet gears, a ring gear,and a planet carrier on which the planet gears are mounted. The gearboxhas an overall gear mesh stiffness. A gear mesh to fan shaft stiffnessratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{fan}\mspace{14mu}{shaft}}$is in the range from 0.3 to 1.6. The method comprises operating the gasturbine engine to provide propulsion under cruise conditions.

According to a twentieth aspect, there is provided a method of operationof a gas turbine engine for an aircraft, the engine comprising an enginecore comprising a turbine, a compressor, and a core shaft connecting theturbine to the compressor, the core shaft having an effective lineartorsional stiffness; a fan located upstream of the engine core, the fancomprising a plurality of fan blades; a fan shaft; and a gearboxarranged to receive an input from the core shaft and to output drive tothe fan via the fan shaft so as to drive the fan at a lower rotationalspeed than the core shaft. The gearbox is an epicyclic gearboxcomprising a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The gearbox has anoverall gear mesh stiffness. A gear mesh to core shaft stiffness ratioof:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{core}\mspace{14mu}{shaft}}$is greater than or equal to 0.2, and optionally in the range from 0.2 to90 or from 0.2 to 29. The method comprises operating the gas turbineengine to provide propulsion under cruise conditions.

The method of any of the seventeenth to twentieth aspects may furthercomprise driving the gearbox with an input torque of:

-   -   (i) greater than or equal to 10,000 Nm at cruise; and/or    -   (ii) greater than or equal to 28,000 Nm at Maximum Take-Off.

The engine used in the method of any of the seventeenth to twentiethaspects may be the gas turbine engine as described for any of thethirteenth to sixteenth aspects.

According to a twenty-first aspect, there is provided a propulsor for anaircraft, the propulsor comprising a fan comprising a plurality of fanblades; a gearbox; a gearbox support arranged to mount the gearboxwithin the engine; and a power unit for driving the fan via the gearbox.The gearbox is an epicyclic gearbox arranged to receive an input from acore shaft driven by the power unit and to output drive to the fan via afan shaft so as to drive the fan at a lower rotational speed than thecore shaft, and comprises a sun gear, a plurality of planet gears, aring gear, and a planet carrier on which the planet gears are mounted.The gearbox has an overall gear mesh stiffness. The fan shaft, coreshaft, gearbox, and gearbox support together form a transmission havingan effective linear torsional stiffness. A gear mesh to transmissionstiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{\begin{matrix}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}} \\{transmission}\end{matrix}}$is in the range from 0.34 to 11.

According to a twenty-second aspect, there is provided a propulsor foran aircraft, the propulsor comprising a fan comprising a plurality offan blades; a gearbox; a gearbox support arranged to mount the gearboxwithin the engine; and a power unit for driving the fan via the gearbox.The gearbox is an epicyclic gearbox arranged to receive an input from acore shaft driven by the power unit and to output drive to the fan so asto drive the fan at a lower rotational speed than the core shaft, andcomprises a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The gearbox has anoverall gear mesh stiffness. A gear mesh to gearbox support stiffnessratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{\begin{matrix}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}} \\{{gearbox}\mspace{14mu}{support}}\end{matrix}}$is greater than or equal to 6.5×10⁻², and optionally less than or equalto 2.6×10¹.

According to a twenty-third aspect, there is provided a propulsor for anaircraft, the propulsor comprising a fan comprising a plurality of fanblades; a gearbox; a power unit for driving the fan via the gearbox; afan shaft; and a core shaft driven by the power unit. The fan shaft hasan effective linear torsional stiffness. The gearbox is an epicyclicgearbox arranged to receive an input from the core shaft and to outputdrive to the fan via the fan shaft so as to drive the fan at a lowerrotational speed than the core shaft, the gearbox comprising a sun gear,a plurality of planet gears, a ring gear, and a planet carrier on whichthe planet gears are mounted. The gearbox has an overall gear meshstiffness. A gear mesh to fan shaft stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{fan}\mspace{14mu}{shaft}}$is in the range from 0.3 to 1.6.

According to a twenty-fourth aspect, there is provided a propulsor foran aircraft, the propulsor comprising a fan comprising a plurality offan blades; a gearbox; a power unit for driving the fan via the gearbox;a fan shaft; and a core shaft driven by the power unit. The core shafthas an effective linear torsional stiffness. The gearbox is an epicyclicgearbox arranged to receive an input from the core shaft and to outputdrive to the fan via the fan shaft so as to drive the fan at a lowerrotational speed than the core shaft, the gearbox comprising a sun gear,a plurality of planet gears, a ring gear, and a planet carrier on whichthe planet gears are mounted. The gearbox has an overall gear meshstiffness. A gear mesh to core shaft stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{core}\mspace{14mu}{shaft}}$is greater than or equal to 0.2, and optionally in the range from 0.2 to29 or from 0.2 to 90.

The propulsor of any of the twenty-first to twenty-fourth aspects maybe, or have any applicable features of, the gas turbine engine asdescribed for any of the thirteenth to sixteenth aspects.

In other aspects, value ranges for products of the components of thevarious ratios may be specified instead of, or as well as, value rangesfor the ratios. For example:

A gear mesh and transmission stiffness product defined as the overallgear mesh stiffness of the gearbox multiplied by the effective lineartorsional stiffness of the transmission may have a value greater than orequal to 1.6×10¹⁷ N²m⁻², and optionally greater than or equal to3.2×10¹⁷ N²m⁻². In various embodiments, the value may be in the rangefrom 1.6×10¹⁷ to 2.9×10¹⁹ N²m⁻², and optionally in the range from3.2×10¹⁷ to 1.5×10¹⁹ N²m⁻².

A gear mesh and gearbox support stiffness product defined as the overallgear mesh stiffness of the gearbox multiplied by the effective lineartorsional stiffness of the gearbox support may have a value greater thanor equal to 2.0×10¹⁷ N²m⁻², and optionally greater than or equal to9.0×10¹⁷ N²m⁻². In various embodiments, the gear mesh and gearboxsupport stiffness product may be in the range from 2.0×10¹⁷ to 4.1×10¹⁹N²m⁻², and optionally in the range from 9.0×10¹⁷ to 2.1×10¹⁹ N²m⁻².

A gear mesh to fan shaft stiffness product defined as the overall gearmesh stiffness of the gearbox multiplied by the effective lineartorsional stiffness of the fan shaft may have a value greater than orequal to 1.3×10¹⁸N²m⁻², and optionally less than 5.0×10¹⁹ N²m⁻².Optionally the value may be greater than or equal to 1.4×10¹⁸N²m⁻², andfurther optionally less than 3.0λ10¹⁹ N²m⁻².

A gear mesh and core shaft stiffness product defined as the overall gearmesh stiffness of the gearbox multiplied by the effective lineartorsional stiffness of the core shaft may have a value in the range from1.0×10¹⁷ to 3.0×10¹⁹ N²m⁻², and optionally in the range from 4.5×10¹⁷ to9.0×10¹⁸N²m⁻².

According to one such aspect, there is provided a gas turbine engine foran aircraft, the engine comprising an engine core comprising a turbine,a compressor, a gearbox, a gearbox support arranged to mount the gearboxwithin the engine, and a core shaft connecting the turbine to thecompressor; a fan located upstream of the engine core, the fancomprising a plurality of fan blades; and a fan shaft. The gearbox isarranged to receive an input from the core shaft and to output drive tothe fan via the fan shaft so as to drive the fan at a lower rotationalspeed than the core shaft. The gearbox is an epicyclic gearboxcomprising a sun gear, a plurality of planet gears, a ring gear, and aplanet carrier on which the planet gears are mounted. The gearbox has anoverall gear mesh stiffness. The fan shaft, the core shaft, the gearbox,and the gearbox support together form a transmission having an effectivelinear torsional stiffness. A gear mesh and transmission stiffnessproduct defined as the overall gear mesh stiffness of the gearboxmultiplied by the effective linear torsional stiffness of thetransmission is greater than or equal to 1.6×10¹⁷ N²m⁻².

The skilled person would appreciate that method and propulsor aspectsmay be formulated accordingly, and that gas turbine engine, method andpropulsor aspects for the other products listed above may be formulatedaccordingly

In any of the preceding aspects, any one or more of the following mayapply as applicable:

The turbine may be a first turbine, the compressor a first compressor,and the core shaft a first core shaft. The engine core may furthercomprise a second turbine, a second compressor, and a second core shaftconnecting the second turbine to the second compressor. The secondturbine, second compressor, and second core shaft may be arranged torotate at a higher rotational speed than the first core shaft.

The planet carrier may comprise a forward plate and a rearward plate andpins extending therebetween. Each pin may be arranged to have a planetgear mounted thereon. The planet carrier may further comprise lugsextending between the forward and rearward plates, the lugs beingarranged to pass between adjacent planet gears.

The gearbox may comprise an odd number of planet gears, and optionallymay comprise 3, 5 or 7 planet gears.

The fan may have a fan diameter greater than 240 cm and less than orequal to 380 cm, and optionally greater than 300 cm and less than orequal to 380 cm.

The gearbox input shaft may provide a soft mounting for the sun gearsuch that some movement of the sun gear is facilitated. The core shaftmay comprise a more stiff section and a less stiff section, the lessstiff section providing the gearbox input shaft and being arranged tolie between the more stiff section and the sun gear and being arrangedto provide, or to contribute to, the soft mounting of the sun gear.

The rotational speed of the fan at cruise conditions may be less than2500 rpm, and optionally less than 2300 rpm.

The fan may have a fan diameter in the range from 240 cm to 280 cm. Insuch embodiments, the rotational speed of the fan at cruise conditionsmay be in the range of from 1700 rpm to 2500 rpm, and optionally in therange of from 1800 rpm to 2300 rpm,

The fan may have a fan diameter in the range from 330 cm to 380 cm. Insuch embodiments, the rotational speed of the fan at cruise conditionsmay be in the range of from 1200 rpm to 2000 rpm, and optionally in therange of from 1300 rpm to 1800 rpm.

A gear ratio of the gearbox may be in any range defined herein, forexample in the range from 3.2 to 4.5, and optionally from 3.3 to 4.0.

A specific thrust of the engine at cruise may be in the range from 70 to90 NKg⁻¹s.

A bypass ratio at cruise may be in the range from 12.5 to 18; andoptionally from 13 to 16.

For any parameter or ratio of parameters X claimed or disclosed herein,a limit on the values that X can take that is expressed as “X is greaterthan or equal to Y” can alternatively be expressed as “1/X is less thanor equal to 1/Y”. Any of the ratios or parameters defined in the aspectsand statements above may therefore be expressed as “1/X is less than orequal to 1/Y” rather than “X is greater than or equal to Y”. Zero may betaken as the lower bound on the value of 1/X.

Various parameters of the gearbox, and/or of the engine more generally,may be adjusted to allow the engine to meet the specifications of thevarious aspects summarised above. Comments on various such parametersare provided below, with examples of ways in which these may be adjustedprovided later in the description of the components.

With respect to gearbox size, and in particular to ring gear pitchcircle diameter (PCD) as a measure of gearbox size, the inventorappreciated that an optimal PCD may be selected by considering therelationship between improved performance due to improved use of thelever effect for larger gearbox sizes, and the effect of increased dragfor larger gearbox sizes (diminishing returns on the improved levereffect from the larger size above a certain PCD, and increased size andweight of the larger size). Ring gear materials may be selected toensure that a maximum expected torque density for the PCD size would bewell within tolerance limits.

The skilled person would appreciate that gear mesh stiffness may becontrolled by selecting one or more of tooth material and/or tooth size;in particular, there may be a trade-off between bending strength ofteeth and contact strength of teeth. A larger tooth length may allow formore sliding and hence more risk of damage, whilst providing a largercontact area for distribution of load. Fewer teeth, each tooth beingwider and stronger than for a gear of the same size with more teeth, maygenerally be preferred, with an upper limit on tooth size set byconsiderations of sliding and misalignment. The inventor appreciatedthat too low a gear mesh stiffness may allow excessively large torsionvibrations, resulting in deleterious misalignments and potential toothdamage.

The inventor appreciated that, whilst a lower gear mesh stiffness mayoffer benefits for correction of misalignment, reducing the stiffnessbelow the levels described herein may result in deleterious torsionvibrations—in particular, the skilled person would appreciate thattorsional vibrations with low modal frequency have relatively highamplitudes (as the product of amplitude and frequency may be at leastsubstantially constant/they may be at least substantially inverselyproportional) and should be avoided or reduced to avoid excessivedeflections.

One or more of gearbox size, gearbox geometry (including presence orabsence of lugs in the carrier, and the number, size, and/or shape ofany lugs present), and material choice, amongst other factors, may beselected or adjusted to achieve a desired carrier stiffness. Thematerials of which the carrier is made (often steels) may, for example,have a Young's modulus in the range from 100 to 250 GPa, or 105 to 215Gpa, and optionally around 210 Gpa—different grades of steel may beselected to achieve different stiffnesses for the same size andgeometry. For example, steels with a Young's modulus in the range 190 to215 Gpa, titanium alloys with a Young's modulus in the range 105 to 120Gpa, or a metal such as titanium with a Young's modulus of around 110Gpa may be used in various embodiments.

Flexibility of the carrier (effectively the inverse of stiffness) allowschanges in alignment of the gears and bearings—the inventor appreciatedthat a certain amount of flexibility in some places may advantageouslyallow manufacturing misalignments to be corrected in use, that a certainmisalignment may be tolerated, and that a larger misalignment coulddeleteriously affect running of the engine, and discovered variousstiffness relationships to capture the advantages of appropriatestiffness ranges.

One or more of shaft diameter, material(s), and wall thickness may beadjusted so as to obtain shaft stiffnesses in the desired ranges.

As noted elsewhere herein, the present disclosure may relate to a gasturbine engine. Such a gas turbine engine may comprise an engine corecomprising a turbine, a combustor, a compressor, and a core shaftconnecting the turbine to the compressor. Such a gas turbine engine maycomprise a fan (having fan blades) located upstream of the engine core.

The gas turbine engine may comprise a gearbox that receives an inputfrom the core shaft and outputs drive to the fan so as to drive the fanat a lower rotational speed than the core shaft. The input to thegearbox may be directly from the core shaft, or indirectly from the coreshaft, for example via a spur shaft and/or gear. The core shaft mayrigidly connect the turbine and the compressor, such that the turbineand compressor rotate at the same speed (with the fan rotating at alower speed).

The gas turbine engine as described and/or claimed herein may have anysuitable general architecture. For example, the gas turbine engine mayhave any desired number of shafts that connect turbines and compressors,for example one, two or three shafts. Purely by way of example, theturbine connected to the core shaft may be a first turbine, thecompressor connected to the core shaft may be a first compressor, andthe core shaft may be a first core shaft. The engine core may furthercomprise a second turbine, a second compressor, and a second core shaftconnecting the second turbine to the second compressor. The secondturbine, second compressor, and second core shaft may be arranged torotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axiallydownstream of the first compressor. The second compressor may bearranged to receive (for example directly receive, for example via agenerally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example the first core shaft in the example above). For example,the gearbox may be arranged to be driven only by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example only be the first core shaft, and not the second coreshaft, in the example above). Alternatively, the gearbox may be arrangedto be driven by any one or more shafts, for example the first and/orsecond shafts in the example above.

The gearbox may be a reduction gearbox (in that the output to the fan isa lower rotational rate than the input from the core shaft). Any type ofgearbox may be used. For example, the gearbox may be a “planetary” or“star” gearbox, as described in more detail elsewhere herein. Thegearbox may have any desired reduction ratio (defined as the rotationalspeed of the input shaft divided by the rotational speed of the outputshaft), for example greater than 2.5, for example in the range of from 3to 4.2, or 3.2 to 3.8, for example on the order of or at least 3, 3.1,3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The gear ratiomay be, for example, between any two of the values in the previoussentence. Purely by way of example, the gearbox may be a “star” gearboxhaving a ratio in the range of from 3.1 or 3.2 to 3.8. In somearrangements, the gear ratio may be outside these ranges.

In any gas turbine engine as described and/or claimed herein, acombustor may be provided axially downstream of the fan andcompressor(s). For example, the combustor may be directly downstream of(for example at the exit of) the second compressor, where a secondcompressor is provided. By way of further example, the flow at the exitto the combustor may be provided to the inlet of the second turbine,where a second turbine is provided. The combustor may be providedupstream of the turbine(s).

The or each compressor (for example the first compressor and secondcompressor as described above) may comprise any number of stages, forexample multiple stages. Each stage may comprise a row of rotor bladesand a row of stator vanes, which may be variable stator vanes (in thattheir angle of incidence may be variable). The row of rotor blades andthe row of stator vanes may be axially offset from each other.

The or each turbine (for example the first turbine and second turbine asdescribed above) may comprise any number of stages, for example multiplestages. Each stage may comprise a row of rotor blades and a row ofstator vanes. The row of rotor blades and the row of stator vanes may beaxially offset from each other.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0% spanposition, to a tip at a 100% span position. The ratio of the radius ofthe fan blade at the hub to the radius of the fan blade at the tip maybe less than (or on the order of) any of: 0.4, 0.39, 0.38, 0.37, 0.36,0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. Theratio of the radius of the fan blade at the hub to the radius of the fanblade at the tip may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 0.28 to 0.32. These ratios maycommonly be referred to as the hub-to-tip ratio. The radius at the huband the radius at the tip may both be measured at the leading edge (oraxially forwardmost) part of the blade. The hub-to-tip ratio refers, ofcourse, to the gas-washed portion of the fan blade, i.e. the portionradially outside any platform.

The radius of the fan may be measured between the engine centreline andthe tip of a fan blade at its leading edge. The fan diameter (which maysimply be twice the radius of the fan) may be greater than (or on theorder of) any of: 220 cm, 230 cm, 240 cm, 250 cm (around 100 inches),260 cm, 270 cm (around 105 inches), 280 cm (around 110 inches), 290 cm(around 115 inches), 300 cm (around 120 inches), 310 cm, 320 cm (around125 inches), 330 cm (around 130 inches), 340 cm (around 135 inches), 350cm, 360 cm (around 140 inches), 370 cm (around 145 inches), 380 (around150 inches) cm, 390 cm (around 155 inches), 400 cm, 410 cm (around 160inches) or 420 cm (around 165 inches). The fan diameter may be in aninclusive range bounded by any two of the values in the previoussentence (i.e. the values may form upper or lower bounds), for examplein the range of from 240 cm to 280 cm or 330 cm to 380 cm.

The rotational speed of the fan may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 2500 rpm, for example less than 2300 rpm.Purely by way of further non-limitative example, the rotational speed ofthe fan at cruise conditions for an engine having a fan diameter in therange of from 220 cm to 300 cm (for example 240 cm to 280 cm or 250 cmto 270 cm) may be in the range of from 1700 rpm to 2500 rpm, for examplein the range of from 1800 rpm to 2300 rpm, for example in the range offrom 1900 rpm to 2100 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for anengine having a fan diameter in the range of from 330 cm to 380 cm maybe in the range of from 1200 rpm to 2000 rpm, for example in the rangeof from 1300 rpm to 1800 rpm, for example in the range of from 1400 rpmto 1800 rpm.

In use of the gas turbine engine, the fan (with associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades 13 on the flow results in an enthalpy rise HI of the flow. A fantip loading may be defined as dH/U_(tip) ², where dH is the enthalpyrise (for example the 1-D average enthalpy rise) across the fan andU_(tip) is the (translational) velocity of the fan tip, for example atthe leading edge of the tip (which may be defined as fan tip radius atleading edge multiplied by angular speed). The fan tip loading at cruiseconditions may be greater than (or on the order of) any of: 0.28, 0.29,0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4. Thefan tip loading may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 0.28 to 0.31, or 0.29 to 0.3.

Gas turbine engines in accordance with the present disclosure may haveany desired bypass ratio, where the bypass ratio is defined as the ratioof the mass flow rate of the flow through the bypass duct to the massflow rate of the flow through the core at cruise conditions. In somearrangements the bypass ratio may be greater than (or on the order of)any of the following: 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5,15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20. The bypass ratiomay be in an inclusive range bounded by any two of the values in theprevious sentence (i.e. the values may form upper or lower bounds), forexample in the range of form 12 to 16, 13 to 15, or 13 to 14. The bypassduct may be substantially annular. The bypass duct may be radiallyoutside the core engine. The radially outer surface of the bypass ductmay be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/orclaimed herein may be defined as the ratio of the stagnation pressureupstream of the fan to the stagnation pressure at the exit of thehighest pressure compressor (before entry into the combustor). By way ofnon-limitative example, the overall pressure ratio of a gas turbineengine as described and/or claimed herein at cruise may be greater than(or on the order of) any of the following: 35, 40, 45, 50, 55, 60, 65,70, 75. The overall pressure ratio may be in an inclusive range boundedby any two of the values in the previous sentence (i.e. the values mayform upper or lower bounds), for example in the range of from 50 to 70.

Specific thrust of an engine may be defined as the net thrust of theengine divided by the total mass flow through the engine. At cruiseconditions, the specific thrust of an engine described and/or claimedherein may be less than (or on the order of) any of the following: 110Nkg⁻¹s, 105 Nkg⁻¹s, 100 Nkg⁻¹s, 95 Nkg⁻¹s, 90 Nkg⁻¹s, 85 Nkg⁻¹s or 80Nkg⁻¹s. The specific thrust may be in an inclusive range bounded by anytwo of the values in the previous sentence (i.e. the values may formupper or lower bounds), for example in the range of from 80 Nkg⁻¹ s to100 Nkg⁻¹s, or 85 Nkg⁻¹s to 95 Nkg⁻¹s. Such engines may be particularlyefficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have anydesired maximum thrust. Purely by way of non-limitative example, a gasturbine as described and/or claimed herein may be capable of producing amaximum thrust of at least (or on the order of) any of the following:160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN,450 kN, 500 kN, or 550 kN. The maximum thrust may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds). Purely by way of example, agas turbine as described and/or claimed herein may be capable ofproducing a maximum thrust in the range of from 330 kN to 420 kN, forexample 350 kN to 400 kN. The thrust referred to above may be themaximum net thrust at standard atmospheric conditions at sea level plus15 degrees C. (ambient pressure 101.3 kPa, temperature 30 degrees C.),with the engine static.

In use, the temperature of the flow at the entry to the high pressureturbine may be particularly high. This temperature, which may bereferred to as TET, may be measured at the exit to the combustor, forexample immediately upstream of the first turbine vane, which itself maybe referred to as a nozzle guide vane. At cruise, the TET may be atleast (or on the order of) any of the following: 1400K, 1450K, 1500K,1550K, 1600K or 1650K. The TET at cruise may be in an inclusive rangebounded by any two of the values in the previous sentence (i.e. thevalues may form upper or lower bounds). The maximum TET in use of theengine may be, for example, at least (or on the order of) any of thefollowing: 1700K, 1750K, 1800K, 1850K, 1900K, 1950K or 2000K. Themaximum TET may be in an inclusive range hounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 1800K to 1950K. The maximumTET may occur, for example, at a high thrust condition, for example at amaximum take-off (MTO) condition.

As used herein, a maximum take-off (MTO) condition has the conventionalmeaning. Maximum take-off conditions may be defined as operating theengine at International Standard Atmosphere (ISA) sea level pressure andtemperature conditions +15° C. at maximum take-off thrust at end ofrunway, which is typically defined at an aircraft speed of around 0.25Mn, or between around 0.24 and 0.27 Mn. Maximum take-off conditions forthe engine may therefore be defined as operating the engine at a maximumtake-off thrust (for example maximum throttle) for the engine atInternational Standard Atmosphere (ISA) sea level pressure andtemperature +15° C. with a fan inlet velocity of 0.25 Mn.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre. By way of further example at least a part of thefan blade and/or aerofoil may be manufactured at least in part from ametal, such as a titanium based metal or an aluminium based material(such as an aluminium-lithium alloy) or a steel based material. The fanblade may comprise at least two regions manufactured using differentmaterials. For example, the fan blade may have a protective leadingedge, which may be manufactured using a material that is better able toresist impact (for example from birds, ice or other material) than therest of the blade. Such a leading edge may, for example, be manufacturedusing titanium or a titanium-based alloy. Thus, purely by way ofexample, the fan blade may have a carbon-fibre or aluminium based body(such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion,from which the fan blades may extend, for example in a radial direction.The fan blades may be attached to the central portion in any desiredmanner. For example, each fan blade may comprise a fixture which mayengage a corresponding slot in the hub (or disc). Purely by way ofexample, such a fixture may be in the form of a dovetail that may slotinto and/or engage a corresponding slot in the hub/disc in order to fixthe fan blade to the hub/disc. By way of further example, the fan bladesmaybe formed integrally with a central portion. Such an arrangement maybe referred to as a bladed disc or a bladed ring. Any suitable methodmay be used to manufacture such a bladed disc or bladed ring. Forexample, at least a part of the fan blades may be machined from a blockand/or at least part, of the fan blades may be attached to the hub/discby welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may notbe provided with a variable area nozzle (VAN). Such a variable areanozzle may allow the exit area of the bypass duct to be varied in use.The general principles of the present disclosure may apply to engineswith or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have anydesired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26fan blades.

As used herein, cruise conditions have the conventional meaning andwould be readily understood by the skilled person. Thus, for a given gasturbine engine for an aircraft, the skilled person would immediatelyrecognise cruise conditions to mean the operating point of the engine atmid-cruise of a given mission (which may be referred to in the industryas the “economic mission”) of an aircraft to which the gas turbineengine is designed to be attached. In this regard, mid-cruise is thepoint in an aircraft flight cycle at which 50% of the total fuel that isburned between top of climb and start of descent has been burned (whichmay be approximated by the midpoint—in terms of time and/ordistance—between top of climb and start of descent. Cruise conditionsthus define an operating point of the gas turbine engine that provides athrust that would ensure steady state operation (i.e. maintaining aconstant altitude and constant Mach Number) at mid-cruise of an aircraftto which it is designed to be attached, taking into account the numberof engines provided to that aircraft. For example where an engine isdesigned to be attached to an aircraft that has two engines of the sametype, at cruise conditions the engine provides half of the total thrustthat would be required for steady state operation of that aircraft atmid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruiseconditions are defined as the operating point of the engine thatprovides a specified thrust (required to provide—in combination with anyother engines on the aircraft—steady state operation of the aircraft towhich it is designed to be attached at a given mid-cruise Mach Number)at the mid-cruise atmospheric conditions (defined by the InternationalStandard Atmosphere according to ISO 2533 at the mid-cruise altitude).For any given gas turbine engine for an aircraft, the mid-cruise thrust,atmospheric conditions and Mach Number are known, and thus the operatingpoint of the engine at cruise conditions is clearly defined.

Purely by way of example, the forward speed at the cruise condition maybe any point in the range of from Mach 0.7 to 0.9, for example 0.75 to0.85, for example 0.76 to 0.84, for example 0.77 to 0.83, for example0.78 to 0.82, for example 0.79 to 0.81, for example on the order of Mach0.8, on the order of Mach 0.85 or in the range of from 0.8 to 0.85. Anysingle speed within these ranges may be part of the cruise condition.For some aircraft, the cruise conditions may be outside these ranges,for example below Mach 0.7 or above Mach 0.9.

Purely by way of example, the cruise conditions may correspond tostandard atmospheric conditions (according to the International StandardAtmosphere, ISA) at an altitude that is in the range of from 10000m to15000 m, for example in the range of from 10000m to 12000m, for examplein the range of from 10400m to 11600m (around 38000 ft), for example inthe range of from 10500m to 11500m, for example in the range of from10600m to 11400m, for example in the range of from 10700m (around 35000ft) to 11300m, for example in the range of from 10800m to 11200m, forexample in the range of from 10900m to 11100m, for example on the orderof 11000m. The cruise conditions may correspond to standard atmosphericconditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to anoperating point of the engine that provides a known required thrustlevel (for example a value in the range of from 30 kN to 35 kN) at aforward Mach number of 0.8 and standard atmospheric conditions(according to the International Standard Atmosphere) at an altitude of38000 ft (11582m). Purely by way of further example, the cruiseconditions may correspond to an operating point of the engine thatprovides a known required thrust level (for example a value in the rangeof from 50 kN to 65 kN) at a forward Mach number of 0.85 and standardatmospheric conditions (according to the International StandardAtmosphere) at an altitude of 35000 ft (10668m).

In use, a gas turbine engine described and/or claimed herein may operateat the cruise conditions defined elsewhere herein. Such cruiseconditions may be determined by the cruise conditions (for example themid-cruise conditions) of an aircraft to which at least one (for example2 or 4) gas turbine engine may be mounted in order to provide propulsivethrust.

According to an aspect, there is provided an aircraft comprising a gasturbine engine as described and/or claimed herein. The aircraftaccording to this aspect is the aircraft for which the gas turbineengine has been designed to be attached. Accordingly, the cruiseconditions according to this aspect correspond to the mid-cruise of theaircraft, as defined elsewhere herein.

According to an aspect, there is provided a method of operating a gasturbine engine as described and/or claimed herein. The operation may beat the cruise conditions as defined elsewhere herein (for example interms of the thrust, atmospheric conditions and Mach Number).

According to an aspect, there is provided a method of operating anaircraft comprising a gas turbine engine as described and/or claimedherein. The operation according to this aspect may include (or may be)operation at the mid-cruise of the aircraft, as defined elsewhereherein.

Whilst in the arrangements described herein the source of drive for thepropulsive fan is provided by a gas turbine engine, the skilled personwill appreciate the applicability of the gearbox configurationsdisclosed herein to other forms of aircraft propulsor comprisingalternative drive types. For example, the above-mentioned gearboxarrangements may be utilised in aircraft propulsors comprising apropulsive fan driven by an electric motor. In such circumstances, theelectric motor may be configured to operate at higher rotational speedsand thus may have a lower rotor diameter and may be more power-dense.The gearbox configurations of the aforesaid aspects may be employed toreduce the rotational input speed for the fan or propeller to allow itto operate in a more favourable efficiency regime. Thus, according to anaspect, there s provided an electric propulsion unit for an aircraft,comprising an electric machine configured to drive a propulsive fan viaa gearbox, the gearbox and/or its inputs/outputs/supports being asdescribed and/or claimed herein.

The skilled person will appreciate that except where mutually exclusive,a feature or parameter described in relation to any one of the aboveaspects may be applied to any other aspect. Furthermore, except wheremutually exclusive, any feature or parameter described herein may beapplied to any aspect and/or combined with any other feature orparameter described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 is a close up sectional side view of an upstream portion of a gasturbine engine;

FIG. 3 is a partially cut-away view of a gearbox for a gas turbineengine;

FIG. 4 is a schematic diagram illustrating automatic load-shareadjustments;

FIG. 5 is a schematic diagram illustrating torsional stiffness of acantilevered beam;

FIG. 6 is a schematic diagram illustrating gear mesh stiffness;

FIG. 7 is a side view of a carrier illustrating torsional stiffness;

FIG. 8 is a front view of a different carrier from that shown in FIG. 7,illustrating torsional stiffness of the carrier;

FIG. 9 is a front view of the carrier of FIG. 8, illustrating torsionalstiffness;

FIG. 10 is a front/sectional view of a carrier comprising lugs;

FIG. 11 is a schematic diagram illustrating overall gearbox meshstiffness;

FIG. 12 is a sectional side view of an engine, illustrating thetransmission;

FIG. 13 is a sectional side view of an engine, illustrating the coreshaft, and lore particularly the gearbox input shaft;

FIG. 14 is a schematic diagram illustrating the torsional stiffness ofthe gearbox input shaft FIG. 15 is a schematic diagram illustratingconnection of the fan shaft to a star gearbox;

FIG. 16 is a schematic diagram illustrating connection of the fan shaftto a planetary gearbox;

FIG. 17 is a schematic diagram illustrating fan shaft torsionalstiffness in an engine with a star gearbox;

FIG. 18 is a schematic diagram illustrating a portion of an engine witha star gearbox;

FIG. 19 is a graph of displacement against load, illustrating an elasticregion within which stiffnesses of components may be determined;

FIG. 20 includes a side view and a front/radial view of the gearboxsupport illustrating torsional stiffness of the gearbox support; and

FIG. 21 illustrates a method.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 having a principal rotationalaxis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23that generates two airflows: a core airflow A and a bypass airflow B.The gas turbine engine 10 comprises a core 11 that receives the coreairflow A. The engine core 11 comprises, in axial flow series, a lowpressure compressor 14, a high-pressure compressor 15, combustionequipment 16, a high-pressure turbine 17, a low pressure turbine 19 anda core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. Thebypass airflow B flows through the bypass duct 22. The fan 23 isattached to and driven by the low pressure turbine 19 via a shaft 26 andan epicyclic gearbox 30.

In use, the core airflow A is accelerated and compressed by the lowpressure compressor 14 and directed into the high pressure compressor 15where further compression takes place. The compressed air exhausted fromthe high pressure compressor 15 is directed into the combustionequipment 16 where it is mixed with fuel and the mixture is combusted.The resultant hot combustion products then expand through, and therebydrive, the high pressure and low pressure turbines 17, 19 before beingexhausted through the nozzle 20 to provide some propulsive thrust. Thehigh pressure turbine 17 drives the high pressure compressor 15 by asuitable interconnecting shaft 27. The fan 23 generally provides themajority of the propulsive thrust. The epicyclic gearbox 30 is areduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shownin FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26,which is coupled to a sun wheel, or sun gear, 28 of the epicyclic geararrangement 30. Radially outwardly of the sun gear 28 and intermeshingtherewith is a plurality of planet gears 32 that are coupled together bya planet carrier 34. The planet carrier 34 constrains the planet gears32 to precess around the sun gear 28 in synchronicity whilst enablingeach planet gear 32 to rotate about its own axis. The planet carrier 34is coupled via linkages 36 to the fan 23 in order to drive its rotationabout the engine axis 9. Radially outwardly of the planet gears 32 andintermeshing therewith is an annulus or ring gear 38 that is coupled,via linkages 40, to a stationary supporting structure 24.

The linkages 36 may be referred to as a fan shaft 36, the fan shaft 36optionally comprising two or more shaft portions 36 a, 36 b coupledtogether. For example, the fan shaft 36 may comprise a gearbox outputshaft portion 36 a extending from the gearbox 30 and a fan portion 36 bextending between the gearbox output shaft portion and the fan 23. Inthe embodiment shown in FIGS. 1 and 2, the gearbox 30 is a planetarygearbox and the gearbox output shaft portion 36 a is connected to theplanet carrier 34—it may therefore be referred to as a carrier outputshaft 36 a. In star gearboxes 30, the gearbox output shaft portion 36 amay be connected to the ring gear 38 it may therefore be referred to asa ring output shaft 36 a. In the embodiment shown in FIGS. 1 and 2, thefan portion 36 b of the fan shaft 36 connects the gearbox output shaftportion 36 a to the fan 23. The output of the gearbox 30 is thereforetransferred to the fan 23, to rotate the fan, via the fan shaft 36. Inalternative embodiments, the fan shaft 36 may comprise a singlecomponent, or more than two components. Unless otherwise indicated orapparent to the skilled person, anything described with respect to anengine 10 with a star gearbox 30 may equally be applied to an enginewith a planetary gearbox 30, and vice versa.

Note that the terms “low pressure turbine” and “low pressure compressor”as used herein may be taken to mean the lowest pressure turbine stagesand lowest pressure compressor stages not including the fan 23)respectively and/or the turbine and compressor stages that are connectedtogether by the interconnecting shaft 26 with the lowest rotationalspeed in the engine (i.e. not including the gearbox output shaft thatdrives the fan 23). In some literature, the “low pressure turbine” and“low pressure compressor” referred to herein may alternatively be knownas the “intermediate pressure turbine” and “intermediate pressurecompressor”. Where such alternative nomenclature is used, the fan 23 maybe referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox 30 is shown by way of example in greater detail inFIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38comprise teeth about their periphery to intermesh with the other gears.However, for clarity only exemplary portions of the teeth areillustrated in FIG. 3. There are four planet gears 32 illustrated,although it will be apparent to the skilled reader that more or fewerplanet gears 32 may be provided within the scope of the claimedinvention. Practical applications of a planetary epicyclic gearbox 30generally comprise at least three planet gears 32.

The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3is of the planetary type, in that the planet carrier 34 is coupled to anoutput shaft via linkages 36, with the ring gear 38 fixed. However, anyother suitable type of epicyclic gearbox 30 may be used. By way offurther example, the epicyclic gearbox 30 may be a star arrangement, inwhich the planet carrier 34 is held fixed, with the ring (or annulus)gear 38 allowed to rotate. In such an arrangement the fan 23 is drivenby the ring gear 38. By way of further alternative example, the gearbox30 may be a differential gearbox in which the ring gear 38 and theplanet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in FIGS. 2 and 3 is byway of example only, and various alternatives are within the scope ofthe present disclosure. Purely by way of example, any suitablearrangement may be used for locating the gearbox 30 in the engine 10and/or for connecting the gearbox 30 to the engine 10. By way of furtherexample, the connections (such as the linkages 36, 40 in the FIG. 2example) between the gearbox 30 and other parts of the engine 10 (suchas the input shaft 26, the output shaft and the fixed structure 24) mayhave any desired degree of stiffness or flexibility. By way of furtherexample, any suitable arrangement of the bearings between rotating andstationary parts of the engine (for example between the input and outputshafts from the gearbox and the fixed structures, such as the gearboxcasing) may be used, and the disclosure is not limited to the exemplaryarrangement of FIG. 2. For example, where the gearbox 30 has a stararrangement (described above), the skilled person would readilyunderstand that the arrangement of output and support linkages andbearing locations would typically be different to that shown by way ofexample in FIG. 2.

Accordingly, the present disclosure extends to a gas turbine enginehaving any arrangement of gearbox styles (for example star orplanetary), support structures, input and output shaft arrangement, andbearing locations.

Optionally, the gearbox may drive additional and/or alternativecomponents (e.g. the intermediate pressure compressor and/or a boostercompressor).

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. For example, such engines may havean alternative number of compressors and/or turbines and/or analternative number of interconnecting shafts. By way of further example,the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20meaning that the flow through the bypass duct 22 has its own nozzle 18that is separate to and radially outside the core engine nozzle 20.However, this is not limiting, and any aspect of the present disclosuremay also apply to engines in which the flow through the bypass duct 22and the flow through the core 11 are mixed, or combined, before (orupstream of) a single nozzle, which may be referred to as a mixed flownozzle. One or both nozzles (whether mixed or split flow) may have afixed or variable area. Whilst the described example relates to aturbofan engine, the disclosure may apply, for example, to any type ofgas turbine engine, such as an open rotor (in which the fan stage is notsurrounded by a nacelle) or turboprop engine, for example.

The geometry of the gas turbine engine 10, and components thereof, isdefined by a conventional axis system, comprising an axial direction(which is aligned with the rotational axis 9), a radial direction (inthe bottom-to-top direction in FIG. 1), and a circumferential direction(perpendicular to the page in the FIG. 1 view). The axial, radial andcircumferential directions are mutually perpendicular.

In the described arrangement, the carrier 34 comprises two plates 34 a,34 b; in particular a forward plate 34 a and a rearward plate 34 b. Eachplate 34 a, 34 b extends in a radial plane, with the forward plate 34 alying further forward in the engine 10/closer to the fan 23 than therearward plate 34 b.

The carrier 34 may take any suitable form. For example, the carrier mayor may not be symmetric about its axial mid-point. Purely by way ofexample, in the described arrangement, the carrier 34 is not symmetricabout its axial mid-point, but rather the rearward plate 34 b is stifferthan the forward plate 34 a (for example by 50 to 300%) to compensatefor an asymmetric torque variation across the gearbox 30. In someembodiments, no forward plate 34 a may be provided, or only a smallerforward plate 34 a. In some embodiments, the plates 34 a, 34 b of thecarrier 34 may have equal stiffnesses (for example, in various planetarygearbox arrangements; stiffer rearward plates 34 b may be preferred insome star gearbox arrangements).

A plurality of pins 33 extend across the carrier 34 (between the forwardand rearward plates 34 a,b in the arrangement being described), asshown, for example, in FIGS. 7 to 10. The pins 33 form a part of thecarrier 34. Each pin 33 has a planet gear 34 mounted thereon.

The carrier stiffness in a region at each of the front and rear ends ofeach pin 33 is arranged to be relatively low in the embodiments beingdescribed, to facilitate a more even load distribution; i.e. to improveload-share factor. This may be described as soft mountings for each pin33. The soft mountings 33 a, 33 b may allow some movement of the pins 33relative to each other, and relative to the carrier plates 34 a, 34 b,so allowing differences between planet gears 34, or other manufacturingdefects, to be accommodated without a significant difference in loadbetween different planet gears 34. In various embodiments, the softmountings 34 a, 34 b may be provided by a portion of the pin 33, by aseparate component, and/or by a portion of the respective carrier plate34 a, 34 b.

The soft mountings 34 a, 34 b may be designed to accommodate movementsto address one or more of carrier bearing location accuracy andclearance, planet pin runout of bearing surface to mounting feature(s),planet gear teeth to bearing runout, planet gear teeth spacing andthickness variation/manufacturing tolerances, sun gear teeth spacing andthickness variation/manufacturing tolerances, and/or gearbox input shaftmainline bearing location accuracy and clearance, or the likes. Forexample, in various embodiments the soft mountings 34 a, 34 b may bearranged to allow around 500 μm of pin movement.

Pin size, design and/or material may be adjusted to provide appropriatestiffnesses to the carrier 34. In some arrangements, such as that shownin FIG. 10, lugs 34 c are provided, extending between the carrier plates34 a, 34 b and past the planet gears 32. The presence/absence of lugs 34c, and the number, shape, and/or material(s) of the lug(s) may vary invarious embodiments, and may be adjusted to provide appropriatestiffnesses to the carrier 34.

Use of flexibility within the gearbox 30 to improve load-share isillustrated schematically in FIG. 4, which shows a planetary gearbox 30with three planet gears 32 a, 32 b, 32 c. In this example, the sun gear28 is slightly off-centre with respect to the ring gear 38, and inparticular is closer to two planet gears 32 a and 32 b than it is to thethird planet gear 32 c. In the schematic example shown, there is nocontact between the third planet gear 32 c, leaving the other two planetgears 32 a, 32 b to take 50% of the load each, rather than around 33%.This relatively extreme example is provided for ease of referenceonly—in reality, situations in which contact with one planet gear 32 cis reduced, but not completely eliminated, would be more likely, forexample leading to a percentage load-share of 20:40:40 or 26:37:37 or31:34:34 or the likes rather than the ideal even load share of ⅓:⅓:⅓(i.e. 33:33:33 as a percentage load share, rounded to the nearestinteger).

In the example shown in FIG. 4, each of the two planet gears 32 a, 32 bin contact with the sun gear 28 exerts a force F_(a), F_(h) on the sungear 28. The resultant force, F_(R), on the sun gear 28 pushes the sungear 28 towards the third planet gear 32 c, so re-establishing contactand making the load-share between planets 32 more even. A soft mountingof the sun gear 28/flexibility in the core input shaft 26 facilitatesthis re-balancing. Such soft mountings of the sun gear 28 may bedesigned to accommodate movements to address one or more of carrierhearing location accuracy and clearance, planet and/or sun gear teethspacing and thickness variation/manufacturing tolerances, and/or gearboxinput shaft mainline bearing location accuracy and clearance, or thelikes. For example, in various embodiments such a soft mounting may bearranged to allow around 1000 μm of sun gear movement.

The skilled person would appreciate that a similar effect would apply ifone of the planet gears 32 were closer to the sun gear 28 than theothers; pushing the relevant planet gear 32 back towards the ring gear38, or if one of the planet gears 32 were larger or smaller than theothers. Soft mounting of the pins 33/flexibility in the carrier 34facilitates this re-balancing. Having an odd number of planet gears 32(e.g. 3, 5 or 7 planet gears) may facilitate this automaticre-distribution of load-share.

Small variations between planet gears 32 and/or misalignments of pins 33or shafts 26 may therefore be accommodated by flexibility within thegearbox 30.

The following general definitions of stiffnesses and other parametersmay be used herein:

Torsional Stiffness FIG. 5 illustrates the definition of the torsionalstiffness of a shaft 401 or other body. A torque, τ, applied to the freeend of the beam causes a rotational deformation, θ (e.g. twist) alongthe length of the beam. The torsional stiffness is the torque appliedfor a given angle of twist i.e. τ/θ. The torsional stiffness has SIunits of Nm/rad.

An effective linear torsional stiffness may be determined for acomponent having a given radius. The effective linear torsionalstiffness is defined in terms of an equivalent tangential force appliedat a point on that radius (with magnitude of torque divided by theradius) and the distance δ (with magnitude of the radius multiplied byθ) moved by a point corresponding to the rotational deformation θ of thecomponent.

Gearbox Diameter

As used herein, gearbox diameter is the diameter of the ring gear 38,and more specifically the pitch circle diameter (PCD) of the ring gear38. The skilled person would appreciate that the ring gear diameterlimits a minimum diameter of the gearbox 30, and is representative ofthe gearbox size. The size and shape of a gearbox casing outside of thering gear diameter may vary depending on materials, required strengths,available space, auxiliary system locations and the likes. The PCD ofthe ring gear 38 is therefore taken as a more meaningful andtransferable measure of the size of the gearbox 30 than an extent of acasing.

The pitch circle of a gear is an imaginary circle that rolls withoutslipping with the pitch circle of any other gear with which the firstgear is meshed. The pitch circle passes through the points where theteeth of two gears meet as the meshed gears rotate—the pitch circle of agear generally passes through a mid-point of the length of the teeth ofthe gear. The PCD can be roughly estimated by taking the average of thediameter between tips of the gear teeth and the diameter between basesof the gear teeth. In various embodiments the PCD of the ring gear 38,which may also be thought of as a diameter of the gearbox 30, may bearound 0.55 to 1.2 m, and optionally in the range from 0.57 to 1.0 m.

Gear Mesh Stiffness

A gear mesh stiffness is defined as the resistance to deformation causedby the contact force acting on the teeth of the gears along the line ofaction of the contact force. The concept of the gear mesh stiffness isillustrated in FIG. 6, which shows two gears 402, 403 which meshtogether so that rotation of the first gear 402 drives rotation of thesecond 403. The contact force between them acts along a line of action404. The line of action 404 is a common tangent to the base circle 405,406 of both of the gears 402, 403. The base circles 405, 406 are definedas the circles from which the involute shape of the teeth is defined aswould be understood by the skilled person. The gear mesh stiffness isexpressed as a linear stiffness along the line of action of the contactforce, and is taken as an averaged value over the revolution of at leastone of the respective gears 402, 403 (optionally a full revolution ofthe gear with most teeth in embodiments in which numbers of teeth arenot equal), and optionally over a full cycle of the gearbox back to itsinitial position.

Gear mesh stiffness is a standard parameter widely used in the field ofgearboxes, and would be understood by the skilled person.

Gear mesh stiffness is assessed in isolation from the carrier 34—thecarrier 34 is treated as being rigid (infinitely stiff)/the stiffness ofthe carrier 34 is ignored, so as to assess the contribution to stiffnessfrom the gear meshes only. Gear mesh stiffness of a pair of gears, orthe overall gear mesh stiffness of a gearbox 30, can be thought of asresistance to movement when the output shaft 36 is held stationarywhilst the input shaft 26 is rotated.

More specific definitions of stiffnesses relating to embodimentsdescribed herein are provided below for ease of understanding.

Carrier Torsional Stiffness

The planet carrier 34 holds the planet gears 32 in place. In variousarrangements including the embodiment being described, the planetcarrier 34 comprises a forward plate 34 a and a rearward plate 34 b, andpins 33 extending between the plates, as illustrated in FIGS. 7 to 10.The pins 33 are arranged to be parallel to the engine axis 9. Inalternative embodiments, a plate 34 b may be provided on only oneside—no plate or only a partial plate may be provided on the other sideof the carrier 34. In the embodiment shown in FIG. 10, the carrier 34additionally comprises lugs 34 c, which may also be referred to aswedges or a web, between the forward and rearward plates 34 a, 34 b. Thelugs 34 c may increase the overall stiffness of the carrier 34.

The stiffness of the carrier 34 is selected to be relatively high toreact centrifugal forces and/or to maintain gear alignment. The skilledperson would appreciate that stiffness is a measure of the displacementthat results from any applied forces or moments, and may not relate tostrength of the component. Hence to react a high load, any stiffness isacceptable so long as the resulting displacement is tolerable. How higha stiffness is desired to keep a displacement within acceptable limitstherefore depends on position and orientation of the gears, which isgenerally referred to as gear alignment (or mis-alignment).

Carrier torsional stiffness is a measure of the resistance of thecarrier 34 to an applied torque, τ, as illustrated in FIG. 7 (axialcross-section) and FIGS. 8 to 10 (radial cross-section). The axis of thetorque is parallel to the engine axis 9. The diagonal lining of theplate 34 b at the rearward end of the carrier 30 in FIG. 7 indicatesthat plate 34 b being treated as rigid and non-rotating (as for acantilever beam mounting). In embodiments with only one plate 34 a, theends of the pins 33 (and of the lugs 34 c if present) further from thesingle plate 34 a are held in place instead.

The torque, τ, is applied to the carrier 34 (at the position of theaxial mid-point of the forward plate 34 a) and causes a rotationaldeformation, θ (e.g. twist) along the length of the carrier 34. Thetwist causes the carrier 34 to “wind up” as the ends of the pins 33 (andof the lugs 34 c if present) are held at a fixed radius on the carrierplates 34 a, 34 b. The angle through which a point on an imaginarycircle 902 on the forward plate 34 a passing through the longitudinalaxis of each pin 33 moves is θ, where θ is the angle measured inradians. The imaginary circle 902 may be referred to as the pin pitchcircle diameter (pin PCD). In various embodiments, the pin PCD may be inthe range from 0.38 to 0.65 m, for example being equal to 0.4 m or 0.55m. An effective linear torsional stiffness can therefore be defined forthe carrier 34 as described above, using the radius r of the imaginarycircle 902 (e.g. as illustrated in FIG. 8).

In various embodiments, the torsional stiffness of the carrier 34 isgreater than or equal to 1.60×10⁸ Nm/rad, and optionally greater than orequal to 2.7×10⁸ Nm/rad. In some embodiments, for example in embodimentsin which the fan diameter is in the range from 240 to 280 cm, thetorsional stiffness of the carrier 34 may be greater than or equal to1.8×10⁸ Nm/rad, and optionally may be greater than or equal to 2.5×10⁸Nm/rad (and optionally may be equal to 4.83×10⁸ Nm/rad). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the torsional stiffness of the carrier 34may greater than or equal to 6.0×10⁸ Nm/rad and optionally may begreater than or equal to 1.1×10⁹ Nm/rad (and optionally may be equal to2.17×10⁹ Nm/rad).

In various embodiments, the torsional stiffness of the carrier 34 is inthe range from 1.60×10⁸ to 1.00×10¹¹ Nm/rad, and optionally in the rangefrom 2.7×10⁸ to 1×10¹⁰ Nm/rad. In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280cm, the torsional stiffness of the carrier 34 may be in the range from1.8×10⁸ to 4.8×10⁹ Nm/rad, and optionally may be in the range from2.5×10⁸ to 6.5×10⁸ Nm/rad (and optionally may be equal to 4.83×10⁸Nm/rad). In some embodiments, for example in embodiments in which thefan diameter is in the range from 330 to 380 cm, the torsional stiffnessof the carrier 34 may be in the range from 6.0×10⁸ to 2.2×10¹⁰ Nm/radand optionally may be in the range from 1.1×10⁹ to 3.0×10⁹ Nm/rad (andoptionally may be equal to 2.17×10⁹ Nm/rad).

In various embodiments, the effective linear torsional stiffness of thecarrier 34 may be greater than or equal to 7.00×10⁹ N/m, and optionallygreater than or equal to 9.1×10⁹ N/m. In some embodiments, for examplein embodiments in which the fan diameter is in the range from 240 to 280cm, the effective linear torsional stiffness of the carrier 34 may begreater than or equal to 7.70×10⁹ N/m. In other such embodiments, theeffective linear torsional stiffness of the carrier 34 may be greaterthan or equal to 9.1×10⁹ N/m, optionally greater than or equal to1.1×10¹⁰ N/m (and optionally may be equal to 1.26×10¹⁰ N/m). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the effective linear torsional stiffnessof the carrier 34 may be greater than or equal to 1.2×10¹⁰ N/m andoptionally may be greater than or equal to 2.1×10¹⁰ N/m (and optionallymay be equal to 2.88×10¹⁰ N/m).

In various embodiments, the effective linear torsional stiffness of thecarrier 34 may be in the range from 7.00×10⁹ to 1.20×10¹¹ N/m, andoptionally in the range from 9.1×10⁹ to 8.0×10¹⁰ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the effective linear torsional stiffnessof the carrier 34 may be in the range from 9.1×10⁹ to 6.0×10¹⁰ N/m, andoptionally may be in the range from 7×10⁹ to 2×10¹⁰ N/m, or from 8.5×10⁹to 2.0×10¹⁰ N/m (and optionally may be equal to 1.26×10¹⁰ N/m). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the effective linear torsional stiffnessof the carrier 34 may be in the range from 1.2×10¹⁰ to 1.2×10¹¹ N/m andoptionally may be in the range from 1.0×10¹⁰ to 5.0×10¹⁰ N/m (andoptionally may be equal to 2.88×10¹⁰ N/m).

The torsional stiffness of the carrier 34 may be controlled so as to bewithin a desired range by adjusting one or more parameters, includingcarrier material(s), carrier geometry, and the presence or absence oflugs.

Gear Mesh Stiffnesses

As shown in FIG. 3, the planet gears 32 engage with both the sun gear 28and the ring gear 38. The epicyclic gearbox 30 therefore has a gear meshstiffness between the planet gears 32 and the ring gear 38 and a gearmesh stiffness between the planet gears 32 and the sun gear 28, witheach gear mesh stiffness being defined in the standard way describedabove. Each planet gear 32 has fewer teeth than the ring gear 38. In theembodiment being described, the gear mesh stiffness between the planetgears 32 and the ring gear 38 is taken as an averaged value over onefull revolution of:

-   -   (i) for a star gearbox 30: the ring gear 38, the planet carrier        34 being stationary; or    -   (ii) for a planetary gearbox 30: the planet carrier 34, the ring        gear 38 being stationary.

Averaging over a full rotation may allow any asymmetries in the gears(e.g. due to manufacturing tolerance) to be accounted for. Inalternative embodiments, the averaged value over a full cycle of thegearbox back to its initial position, over a single rotation of a planetgear 32, or just over a single tooth interaction (i.e. over the rollangle change from a selected tooth making contact with the opposing gearand then losing contact with the opposing gear), may be used instead.Further, in the arrangement being described, an average of the valuesobtained for each planet gear 32 is used. The skilled person wouldappreciate that the values for each planet 32 should be the same withintolerances, with any significant deviations suggesting a manufacturingerror or damaged gear.

In various embodiments, the gear mesh stiffness between the planet gears32 and the ring gear 38 is greater than or equal to 1.40×10⁹ N/m, andoptionally greater than or equal to 2.45×10⁹ N/m. In some embodiments,for example in embodiments in which the fan diameter is in the rangefrom 240 to 280 cm, the gear mesh stiffness between the planet gears 32and the ring gear 38 may be greater than or equal to 2.4×10⁹ N/m, andoptionally greater than or equal to 2.5×10⁹, and optionally may be equalto 2.62×10⁹ N/m. In some embodiments, for example in embodiments inwhich the fan diameter is in the range from 330 to 380 cm, gear meshstiffness between the planet gears 32 and the ring gear 38 may begreater than or equal to 2.8×10⁹ N/m, and optionally greater than orequal to 3.2×10⁹ (and optionally may be equal to 3.50×10⁹ N/m).

In various embodiments, the gear mesh stiffness between the planet gears32 and the ring gear 38 is in the range from 1.40×10⁹ to 2.00×10¹⁰ N/m,and optionally in the range from 2.45×10⁹ to 1.05×10¹⁰ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the gear mesh stiffness between the planetgears 32 and the ring gear 38 may be in the range from 2.4×10⁹ to7.5×10⁹ N/m, and optionally in the range from 2.5×10⁹ to 5.5×10 N/m, andoptionally may be equal to 2.62×10⁹ N/m. In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, gear mesh stiffness between the planet gears 32 and thering gear 38 may be in the range from 2.8×10⁹ to 1.05×10¹⁰ N/m, andoptionally in the range from 3.2×10⁹ to 6.5×10⁹ N/m (and optionally maybe equal to 3.50×10⁹ N/m).

The planet to ring gear mesh stiffness may be controlled to be withinthe desired range by adjusting parameters such as tooth size andmaterials, as for other gear mesh stiffnesses.

In the embodiment being described, the gear mesh stiffness between theplanet gears 32 and the sun gear 28 is taken as an averaged value overone full revolution of:

-   -   (i) if the sun gear 28 has more teeth than each planet gear 32,        the sun gear 28; or    -   (ii) if each planet gear 32 has more teeth than the sun gear 28,        the planet gear 32.

The skilled person would appreciate that gear mesh stiffness may varydepending on how many teeth on each gear are in contact at the time, andalso on which portion(s) of a given tooth are in contact with a giventooth on a meshed gear at a time (e.g. tip to root, middle to middle, orroot to tip)—these generally vary with roll angle, and a step change ingear mesh stiffness may be observed as contact with one tooth is lostand/or contact with another tooth gained. Using helical gear teeth mayhelp to smooth any such step change due to different parts of thehelical tooth loosing/gaining contact with the opposing tooth as rollangle changes, but variation, and often discontinuities, over the toothinteraction process are generally expected. At a minimum, the gear meshstiffnesses used are therefore averaged over at least one full toothinteraction process (i.e. over the roll angle change from a selectedtooth making contact with the opposing gear and then losing contact withthe opposing gear). Averaging over a full rotation of a gear, oroptionally of the entire gearbox 30, may allow any asymmetries in thegears/variations between teeth on the same gear (e.g. due tomanufacturing tolerance) to be accounted for. In some embodiments, theaveraged value over a full cycle of the gearbox back to its initialposition may be used instead.

In various embodiments, the gear mesh stiffness between the planet gears32 and the sun gear 28 is greater than or equal to 1.20×10⁹ N/m, andoptionally greater than or equal to 2.0×10⁹ N/m. In some embodiments,for example in embodiments in which the fan diameter is in the rangefrom 240 to 280 cm, the gear mesh stiffness between the planet gears 32and the sun gear 28 may be greater than or equal to 1.9×10⁹ N/m, andoptionally greater than or equal to 2.0×10⁹ N/m, and optionally may beequal to 2.16×10⁹ N/m. In some embodiments, for example in embodimentsin which the fan diameter is in the range from 330 to 380 cm, gear meshstiffness between the planet gears 32 and the sun gear 28 may be greaterthan or equal to 2.3×10⁹ N/m, and optionally greater than or equal to2.8×10⁹ N/m, and optionally may be equal to 3.04×10⁹N/m.

In various embodiments, the gear mesh stiffness between the planet gears32 and the sun gear 28 is in the range from 1.20×10⁹ to 1.60×10¹⁰ N/m,and optionally in the range from 2.0×10⁹ to 9.5×10⁹ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the gear mesh stiffness between the planetgears 32 and the sun gear 28 may be in the range from 1.9×10⁹ to 6.5×10⁹N/m, and optionally in the range from 2.0×10⁹ to 3.0×10⁹ N/m, andoptionally may be equal to 2.16×10⁹ N/m. In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, gear mesh stiffness between the planet gears 32 and thesun gear 28 may be in the range from 2.3×10⁹ to 9.5×10⁹ N/m, andoptionally in the range from 2.8×10⁹ to 4.0×10⁹ N/m (and optionally maybe equal to 3.04×10⁹ N/m).

The planet to sun gear mesh stiffness may be controlled to be within thedesired range by adjusting parameters such as tooth size and materials,as for other gear mesh stiffnesses.

An overall gear mesh stiffness for the gearbox 30 is also defined. Theoverall gear mesh stiffness, T, of the gearbox 30 for a gearbox 30having N planet gears, where N is an integer greater than or equal totwo, may be defined as:

$\frac{1}{T} = {\frac{1}{\sum\limits_{n = 1}^{N}P_{n}^{S}} + \frac{1}{\sum\limits_{n = 1}^{N}P_{n}^{R}}}$where:P_(n) ^(S) is the gear mesh stiffness between the planet gear 32 and thesun gear 28 for the nth planet gear 32; andP_(n) ^(R) is the gear mesh stiffness between the planet gear 32 and thering gear 38 for the nth planet gear 32.

The sum over the planets (Σ_(n=1) ^(N) P_(n)) may be replaced with Ntimes the appropriate (average) gear mesh stiffness as defined above,for both the sun gear mesh (Σ_(n=1) ^(N) P_(n) ^(S)) and the ring gearmesh (Σ_(n=1) ^(N) P_(n) ^(R)).

In the embodiment being described, the averaged value over a full cycleof the gearbox 30 back to its initial position is used.

The overall gear mesh stiffness of the gearbox 30 is illustrated in FIG.11, which schematically shows the connections between the sun gear 28and the planet carrier 34 (via the planet gears 32, with gear meshstiffnesses P_(n) ^(S)) and the connections between the planet carrier34 and the ring gear 38 (via the planet gears 32, with gear meshstiffnesses P_(n) ^(R)).

In various embodiments, the overall gear mesh stiffness of the gearbox30 is greater than or equal to 1.05×10⁹ N/m, optionally in the rangefrom 1.05×10⁹ to 8.00×10⁹ N/m, and further optionally in the range from1.08×10⁹ to 4.9×10⁹ N/m, or to 3.4×10⁹ N/m. In some embodiments, forexample in embodiments in which the fan diameter is in the range from240 to 280 cm, the overall gear mesh stiffness of the gearbox 30 may bein the range from 1.05×10⁹ to 3.6×10⁹ N/m, and optionally in the rangefrom 1.08×10⁹ to 1.28×10⁹ N/m, and optionally may be equal to 1.18×10⁹N/m. In some embodiments, for example in embodiments in which the fandiameter is in the range from 330 to 380 cm, overall gear mesh stiffnessof the gearbox 30 may be in the range from 1.2×10⁹ to 4.9×10⁹ N/m, andoptionally in the range from 1.4×10⁹ to 2.2×10⁹ N/m (and optionally maybe equal to 1.63×10⁹ N/m).

The skilled person would appreciate that tooth and gearbox dimensions,and gear materials, may be selected as appropriate to obtain a desiredgear mesh stiffness. For example, tooth size may be selected consideringtwo competing factors—a minimum required bending strength of the toothmay set a minimum size for a tooth of a given material, and a maximumallowed amount of slide between teeth may set an upper size limit for atooth. The skilled person would appreciate that larger teeth can resultin more heat generation at the gear mesh, and/or excessive contactbetween meshed gears, which may waste energy and/or increase wear ongears. Having a larger number of smaller teeth (for a given geardiameter), e.g. 80 or more teeth, is therefore generally preferable,with a lower limit being set by a minimum acceptable tooth bendingstrength.

Transmission Torsional Stiffness

Transmission torsional stiffness is a measure of the resistance of thewhole transmission—from the gearbox input shaft 26 to the interface withthe fan 23—to an applied torque, τ, as illustrated in FIG. 12. It may bedescribed as resistance to twisting, or winding, of the shafttransmission. The axis of the moment is parallel to the engine axis 9.

In particular, the transmission may be defined between the bearing 26 cof the core shaft 26 (at or near the rearward end of the gearbox inputshaft 26 a, as described below) and the fan input position, Y, asdefined below. The hearing 26 c (connecting the shaft 26 to thestationary supporting structure 24) and the connection of the gearboxsupport 40 to the stationary supporting structure 24 are held rigidly(non-rotating) as indicated by the diagonally-lined boxes in FIG. 12. Atorque is then applied to the fan shaft 36 at the axial position of thefan input position. For the purpose of measuring the transmissiontorsional stiffness, the gearbox input shaft 26 is held not to rotate atthe location of the bearing 26 c. A rotation angle θ is measured at thefan input position.

The gear mesh stiffnesses are included in the transmission stiffness theblack shading in FIG. 12 indicates components which contribute totransmission stiffness (namely the gearbox input shaft 26, output/fanshaft 36, gearbox 30 and gearbox support 40).

The torque, τ, is applied to the fan shaft 36 (at the fan inputposition, Y) and causes a rotational deformation along the length of thetransmission. The angle through which a point on the fan shaftcircumference at the fan input position moves is θ, where θ is the anglemeasured in radians. An effective linear torsional stiffness cantherefore be defined for the transmission as described above, using theradius, r, of the fan shaft 36. In embodiments in which the radius ofthe fan shaft varies, the radius at the fan input position, Y, may beused.

In various embodiments, the effective linear torsional stiffness of thetransmission is greater than or equal to 1.60×10⁸ N/m, and optionallygreater than or equal to 3.8×10⁸ N/m. In some embodiments, for examplein embodiments in which the fan diameter is in the range from 240 to 280cm, the effective linear torsional stiffness of the transmission may begreater than or equal to 3.8×10⁸ N/m, and optionally may greater than orequal to 4.2×10⁸ N/m (and optionally may be equal to 4.8×10⁸ N/m). Insome embodiments, for example in embodiments in which the fan diameteris in the range from 330 to 380 cm, the effective linear torsionalstiffness of the transmission may be greater than or equal to 3.8×10⁸N/m and optionally may be greater than or equal to 7.7×10⁸ N/m (andoptionally may be equal to 8.2×10⁸ N/m).

In various embodiments, the effective linear torsional stiffness of thetransmission is in the range from 1.60×10⁸ to 3.20×10⁹ N/m, andoptionally in the range from 3.8×10⁸ to 1.9×10⁹ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the effective linear torsional stiffnessof the transmission may be in the range from 3.80×10⁸ to 8.6×10⁸ N/m,and optionally may be in the range from 4.20×10⁸ to 5.4×10⁸ N/m (andoptionally may be equal to 4.8×10⁸ N/m). In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the effective linear torsional stiffness of thetransmission may be in the range from 3.8×10⁸ to 3.2×10⁹ N/m andoptionally may be in the range from 7.7×10⁸ to 9.3×10⁸ N/m (andoptionally may be equal to 8.2×10⁸ N/m).

The torsional stiffness of the transmission may therefore be thought ofas a combined torsional stiffness of the fan shaft 36, the gearbox 30(the overall gearbox mesh stiffness), the core shaft 26 (sun input shaft26), and the gearbox support 40. To adjust the torsional stiffness ofthe transmission to a desired value, the skilled person would appreciatethat any one or more of the parameters discussed for the components ofthe transmission, as described elsewhere herein, may be adjusted asappropriate.

The overall gearbox mesh stiffness is as defined above. The torsionalstiffnesses of the other transmission components may be as definedbelow:

Gearbox Input Shaft Torsional Stiffness

In the arrangement being described, the gearbox input shaft 26 a drivesthe sun gear 28. The gearbox input shaft 26 a may therefore be referredto as a sun input shaft 26 a. The gearbox input shaft 26 a may be a suninput shaft 26 a in star arrangements (as well as planetary). Thegearbox input shaft 26 a may also be referred to as a part of the coreshaft 26—a forward portion 26 a of the core shaft 26 provides the inputto the gearbox 30.

The core shaft 26 therefore comprises a gearbox input shaft 26 a, whichrotates with the rest of the core shaft 26 but may have a differentstiffness from the rest of the core shaft. In the arrangement beingdescribed with respect to FIGS. 1 and 2, the core shaft extends betweenthe turbine 19 and the gearbox 30, connecting the turbine 19 to thecompressor 14, and the turbine and compressor to the gearbox 30. Arearward portion 26 b of the core shaft 26 extends between the turbine19 and the compressor 14, connecting the turbine to the compressor. Aforward portion 26 a extends between the compressor 14 and the gearbox,connecting the turbine and compressor to the gearbox 30. As this forwardportion provides the torque to the gearbox 30, it is referred to as thegearbox input shaft. In the arrangement shown, a bearing 26 c is presenton the core shaft 26 at or near the axial position at which the rearwardportion 26 b meets the gearbox input shaft 26 a.

In some gearboxes 30, the planet carrier 34 may be driven by the coreshaft 26, and more specifically by the gearbox input shaft 26 a, forexample—in such embodiments, the gearbox input shaft 26 a may not be asun input shaft 26 a. However, this may make mounting of the sun gear 28more difficult.

In the described arrangement, the core shaft 26 is divided into twosections as shown in FIG. 13; a first section 26 a (the gearbox inputshaft) extending from the gearbox 30 and connected to the sun gear 28,and a second section 26 b (which may be referred to as a turbine shaft)extending rearwardly from the first section and connected to the turbine19. In the described arrangement, the first section 26 a is designed tohave a lower stiffness than the second section 26 b—the gearbox inputshaft 26 a may therefore provide a soft mounting for the sun gear 28whilst maintaining rigidity elsewhere in the engine 10. In the describedarrangement, the second section 26 b is designed to be effectively rigid(as compared to the stiffness of the first section 26 a). The secondsection 26 b connecting the turbine 19 and the compressor 14 and thegearbox 30 may be referred to as the turbine shaft 26 h. The turbineshaft 26 b is arranged to transmit the torsional loads to drive thecompressor and the gearbox 30, as well as to handle the compressor andturbine axial loads.

In alternative embodiments, the core shaft 26 may not be divided intosections of different stiffness, and may instead have a constantstiffness. In alternative or additional embodiments, the core 26 may bedivided into a larger number of sections.

The core shaft 26 is mounted using a bearing 26 c—the bearing 26 c isthe first bearing on the core shaft 26 axially downstream of the gearbox30. In the described arrangement, the bearing 26 c is on the secondsection 26 b of the shaft 26—in other embodiments, it may be on adifferent, or on the only, shaft section. The stiffnesses of the gearboxinput shaft 26 a are measured holding the bearing 26 c rigid, and takingthe connection of the bearing 26 c to the rest 26 b of the core shaft 26as rigid, such that only the stiffnesses of the first portion 26 a areconsidered (the remainder being treated as effectively rigid). For thepurpose of determining torsional stiffness, the gearbox input shaft 26 ais considered to be free at the end to which the applied torque τ isapplied.

Gearbox input shaft torsional stiffness is a measure of the resistanceof the shaft 26 a to an applied torque, τ, as illustrated in FIG. 14. Itmay be described as resistance to twisting, or winding, of the shaft 26a. The axis of the moment is parallel to the engine axis 9. Thediagonally lined box 402 at the location of the bearing 26 c of theshaft 26 a is shown to indicate the connection to the bearing 26 c/theshaft 26 at the position of the bearing being treated as rigid andnon-rotating (as for a cantilever beam mounting). The shaft 26 a isotherwise treated as a free body (the sun gear-planet gear meshstiffness is not included). A torque, τ, is applied to the shaft 26 a(at the forward position—the position of the axial mid-point of the sungear 28) and causes a rotational deformation, θ (e.g. twist) along thelength of the shaft 26 a. θ is measured at the position of applicationof the torque. As described above, the core shaft 26 is held to benon-rotating at the location of the bearing 26 c, such that the value ofthe twist increases from zero to θ along the length of the first shaftportion 26 a.

In the embodiment shown, the position of the axial mid-point of the sungear 28 is also at or near the forward end of the shaft 26. Inalternative embodiments, the shaft 26 may extend further forward of thesun gear 28; the forward position used for the application of thetorque, force or moment is still taken to be the position of the axialmid-point of the sun gear 28 in such embodiments.

The angle through which a point on the shaft circumference at theforward position moves is θ, where θ is the angle measured in radians.An effective linear torsional stiffness can therefore be defined for thegearbox input shaft 26 a as described above, using the radius, r, of thegearbox input shaft 26 a. In embodiments in which the gearbox inputshaft 26 a varies in radius, the radius of the shaft 26 a at theinterface to the sun gear 28 may be used as the radius r (i.e. theradius at the forward end of the shaft for the embodiment shown).

In various embodiments, the torsional stiffness of the gearbox inputshaft 26 a is greater than or equal to 1.4×10⁶ Nm/radian, and optionallygreater than or equal to 1.6×10⁶ Nm/radian. In some embodiments, forexample in embodiments in which the fan diameter is in the range from240 to 280 cm, the torsional stiffness of the gearbox input shaft may begreater than or equal to 1.4×10⁶ Nm/radian or 2×10⁶ Nm/radian. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the torsional stiffness of the gearboxinput shaft may be greater than or equal to 3×10⁶ Nm/radian or 5×10⁶Nm/radian.

In various embodiments, the torsional stiffness of the gearbox inputshaft 26 a is in the range from 1.4×10⁶ to 2.5×10⁸ Nm/radian, andoptionally in the range from 1.6×10⁶ to 2.5×10⁷ Nm/radian. In someembodiments, for example in embodiments in which the fan diameter s inthe range from 240 to 280 cm, the torsional stiffness of the gearboxinput shaft may be in the range from 1.4×10⁶ to 2.0×10⁷ Nm/radian, andoptionally may be in the range from 1.8×10⁶ to 3.0×10⁶ Nm/radian (andoptionally may be equal to 2.0×10⁶ Nm/radian). In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the torsional stiffness of the gearbox input shaft may bein the range from 3×10⁶ to 1×10⁸Nm/radian and optionally may be in therange from 5×10⁶ to 6×10⁶ Nm/radian (and optionally may be equal to5.7×10⁶ Nm/radian).

In various embodiments, the effective linear torsional stiffness of thegearbox input shaft 26 a is greater than or equal to 4.0×10⁸ N/m, andoptionally greater than or equal to 4.3×10⁸ N/m. In some embodiments,for example in embodiments in which the fan diameter is in the rangefrom 240 to 280 cm, the effective linear torsional stiffness of thegearbox input shaft may be greater than or equal to 4.0×10⁸ N/m or4.4×10⁸ N/m. In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 330 to 380 cm, effective lineartorsional stiffness of the gearbox input shaft may be greater than orequal to 4.3×10⁸ N/m or 6.0×10⁸ N/m.

In various embodiments, effective linear torsional stiffness of thegearbox input shaft is in the range 4.0×10⁸ to 3.0×10¹⁰ N/m, andoptionally in the range from 4.3×10⁸ to 9.0×10⁹ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the effective linear torsional stiffnessof the gearbox input shaft may be in the range from 4.0×10⁸ to 1.5×10¹⁰N/m, and optionally may be in the range from 4.4×10⁸ to 5.4×10⁸ N/m (andoptionally may be equal to 4.9×10⁸ N/m, and optionally 4.92×10⁸ N/m). Insome embodiments, for example in embodiments in which the fan diameteris in the range from 330 to 380 cm, the effective linear torsionalstiffness of the gearbox input shaft may be in the range from 4.3×10⁸ to3.0×10¹⁰ N/m and optionally may be in the range from 5.0×10⁸ or 6.0×10⁸to 8.0×10⁸ N/m (and optionally may be equal to 6.8×10⁸ N/m, andoptionally 6.84×10⁸ N/m).

One or more of gearbox input shaft 26 a material(s), diameter andstructure (e.g. hollow or solid, wall thickness) may be adjusted toachieve a stiffness within the desired range.

Fan Shaft Torsional Stiffness

The fan shaft 36 is defined as the torque transfer component thatextends from the output of the gearbox 30 to the fan input. It thereforeincludes part, or all of any gearbox output shaft and fan input shaftthat may be provided between those points. For the purposes of definingthe stiffness of the fan shaft 36 it is considered to extend between afan input position, Y, and a gearbox output position, X, and to includeall of the torque transfer components between those points. It does nottherefore include any components of the gearbox (e.g. the planet carrieror connecting plate coupled to it) which transmit discrete forces,rather than the fan shaft torque. The gearbox output position (X)therefore may be defined as the point of connection between the fanshaft 36 and the gearbox 30. The fan input position (Y) may be definedas the point of connection between the fan shaft 36 and the fan.

The torsional stiffness of the fan shaft 36 is measured between theforward and rearward ends of the fan shaft; the forward end being theinterface with the fan 23 and the rearward end being the interface withthe gearbox 30.

Fan shaft torsional stiffness is a measure of the resistance of theshaft 36 to an applied torque, τ, as illustrated in FIG. 17. It may bedescribed as resistance to twisting, or winding, of the shaft 36. Theaxis of the moment is parallel to the engine axis 9. Referring to FIGS.15 and 17, where the gearbox 30 is a star gearbox, the gearbox outputposition is defined as the point of connection 702 between the ring gear38 and the fan shaft 36. More specifically, it is the point ofconnection to the annulus of the ring gear 38 (with any connectioncomponent extending from the outer surface of the annulus beingconsidered to be part of the ring gear). Where the point of connectionis formed by an interface extending in a direction having an axialcomponent, the point of connection, X, is considered to be the axialcentreline of that interface as illustrated in FIG. 17. The fan shaft 36includes all torque transmitting components up to the point ofconnection 702 with the ring gear 38. It therefore includes any flexibleportions or linkages 704 of the fan shaft 36 that may be provided, andany connection(s) 706 (e.g. spline connections) between them.

Where the gearbox 30 has a planetary configuration, the gearbox outputposition is again defined as the point of connection between the fanshaft 36 and the gearbox 30. An example of this is illustrated in FIG.16, which shows a carrier comprising a forward plate 34 a and rearwardplate 34 b, with a plurality of pins 33 extending between them and onwhich the planet gears are mounted. The fan shaft 36 is connected to theforward plate 34 a via a spline connection 708. In arrangements such asthis, the gearbox output position is taken as any, point on theinterface between the fan shaft 36 and the forward plate 34 a. Theforward plate 34 a is considered to transmit discrete forces, ratherthan a single torque, and so is taken to be part of the gearbox 30rather than the fan shaft. FIG. 16 shows only one example of a type ofconnection between the fan shaft and planet carrier 34. In embodimentshaving different connection arrangements, the gearbox output position isstill taken to be at the interface between components transmitting atorque (i.e. that are part of the fan shaft) and those transmittingdiscrete forces (e.g. that are part of the gearbox). The splineconnection 708 is only one example of a connection that may be formedbetween the fan shaft 36 and gearbox 30 (i.e. between the fan shaft andthe forward plate 34 b in the presently described embodiment). In otherembodiments, the interface which forms the gearbox output position maybe formed by, for example, a curvic connection, a bolted joint or othertoothed or mechanically dogged arrangement.

The fan input position, Y, is defined as a point on the fan shaft 36 atthe axial midpoint of the interface between the fan 23 and the fan shaft36. In the presently described embodiment, the fan 23 comprises asupport arm 23 a arranged to connect the fan 23 to the fan shaft 36. Thesupport arm 23 a is connected to the fan shaft by a spline coupling 36 athat extends along the length of a portion of the fan shaft 36. The faninput position is defined as the axial midpoint of the spline couplingas indicated by axis Y in FIG. 17. The spline coupling shown in FIG. 17is only one example of a coupling that may form the interface betweenthe fan and fan shaft. In other embodiments, for example, a curvicconnection, a bolted joint or other toothed or mechanically doggedarrangement may be used. The fan input position, Y, may be unaffected bygearbox type.

The fan shaft 36 has a degree of flexibility characterised in part byits torsional stiffness.

The diagonally-lined ring gear 38 in FIG. 17 indicates the ring gear 38being treated as rigid and non-rotating for the purpose of assessingtorsional stiffness. A torque, τ, is applied to the shaft 36 at the faninput position, Y, and causes a rotational deformation, θ (e.g. twist)along the length of the shaft 36. The angle through which a point on theshaft circumference at the fan input position moves is θ, when θ is theangle measured in radians. An effective linear torsional stiffness cantherefore be defined for the fan shaft 36 as described above using theradius, r, of the fan shaft 36. In embodiments in which the fan shaft 36varies in radius, such as the embodiment being described, the radius ofthe shaft 36 at the fan input position may be used as the radius r (i.e.the radius at the forward end of the shaft for the embodiment shown).For the purpose of determining torsional stiffness, the fan shaft 36 isconsidered to be free at the end to which the applied torque z isapplied.

In various embodiments, the torsional stiffness of the fan shaft 36 isequal to or greater than 1.3×10⁷ Nm/rad, and optionally equal to orgreater than 1.4×10⁷ Nm/rad. In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280 cmthe torsional stiffness of the fan shaft 36 may equal to or greater than1.3×10⁷ Nm/radian, and optionally may be equal to or greater than1.4×10⁷ Nm/radian (and optionally may be equal to 1.8×10⁷ Nm/radian). Insome embodiments, for example in embodiments in which the fan diameteris in the range from 330 to 380 cm, the torsional stiffness of the fanshaft 36 may be equal to or greater than 2.5×10⁷ Nm/radian andoptionally may be equal to or greater than 3.5×10⁷ Nm/radian (andoptionally may be equal to 5.2×10⁷ Nm/radian).

In various embodiments, the torsional stiffness of the fan shaft 36 isin the range from 1.3×10⁷ to 2 5×10⁹ Nm/rad, and optionally in the rangefrom 1.4×10⁷ to 3.0×10⁸ Nm/rad. In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280 cmthe torsional stiffness of the fan shaft 36 may be in the range from1.3×10⁷ to 2.0×10⁸ Nm/radian, and optionally may be in the range from1.3×10⁷ to 2.3×10⁷ Nm/radian (and optionally may be equal to 1.8×10⁷Nm/radian). In some embodiments, for example in embodiments in which thefan diameter is in the range from 330 to 380 cm, the torsional stiffnessof the fan shaft 36 may be in the range from 2.5×10⁷ to 2.5×10⁹Nm/radian and optionally may be in the range from 3.5×10⁷ to 7.5×10⁷Nm/radian (and optionally may be equal to 5.2×10⁷ Nm/radian).

In various embodiments, the effective linear torsional stiffness of thefan shaft 36 may be greater than or equal to 1.2×10⁹ N/m, and optionallygreater than or equal to 1.35×10⁹ N/m. In some embodiments, for examplein embodiments in which the fan diameters in the range from 240 to 280cm, the effective linear torsional stiffness of the fan shaft 36 may begreater than or equal to 1.2×10⁹ N/m, and optionally may be greater than1.3×10⁹ Nm/radian (and optionally may be equal to 1.5×10⁹ N/m). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the effective linear torsional stiffnessof the fan shaft 36 may be greater than or equal to 1.5×10⁹ Nina andoptionally may be greater than or equal to 1.8×10⁹ Nm/radian (andoptionally may be equal to 2.1×10⁹ N/m).

In various embodiments, the effective linear torsional stiffness of thefan shaft 36 is in the range from 1.2×10⁹ to 2.0×10¹⁰ N/m, andoptionally in the range from 1.35×10⁹ to 1.0×10¹⁰ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the effective linear torsional stiffnessof the fan shaft 36 may be in the range from 1.2×10⁹ to 1.5×10¹⁰ N/m,and optionally may be in the range from 1.3×10⁹ to 2.3×10⁹ Nm/radian(and optionally may be equal to 1.5×10⁹ N/m). In some embodiments, forexample in embodiments in which the fan diameter s in the range from 330to 380 cm, the effective linear torsional stiffness of the fan shaft 36may be in the range from 1.5×10⁹ to 2.0×10¹⁰ N/m and optionally may bein the range from 1.8×10⁹ to 3.5×10⁹ Nm/radian (and optionally may beequal to 2.1×10⁹ N/m).

One or more of fan shaft 36 material(s), diameter and structure (e.g.hollow or solid, wall thickness) may be adjusted to achieve a stiffnesswithin the desired range.

Gearbox Support Torsional Stiffness

An exemplary embodiment of the gas turbine engine is shown in FIG. 18,which shows a region of the engine core 11 around the gearbox 30 inclose up. The same reference numbers have been used for componentscorresponding to those shown in FIGS. 1 to 3. In the arrangement shownin FIG. 18 the gearbox 30 has a star arrangement, in which the ring gear38 is coupled to the fan shaft 36 and the carrier 34 is held in a fixedposition relative to the static structure 502, 24 of the engine core 11.As noted elsewhere herein, all features and characteristics describedherein may apply to a star gearbox and a planetary gearbox, unlessexplicitly stated otherwise.

The engine core 11 comprises a gearbox support 40 (corresponding to thelinkage described with reference to FIG. 2) arranged to at leastpartially support the gearbox 30 in a fixed position within the engine.The gearbox support is coupled at a first end to the stationarysupporting structure 24 which extends across the core duct carrying thecore airflow A as illustrated in FIG. 18. In the presently describedarrangement, the stationary support structure 24 is or comprises anengine section stator (ESS) that acts as both a structural component toprovide a stationary mounting for core components such as the gearboxsupport 40, and as a guide vane provided to direct airflow from the fan23. In other embodiments, the stationary supporting structure 24 maycomprise a strut extending across the core gas flow path and a separatestator vane provided to direct airflow. In the present embodiment, thegearbox support 40 is coupled at a second end to the planet carrier 34.The gearbox support 40 therefore acts against rotation of the planetcarrier 34 relative to the static structure of the engine core 11. Inembodiments where the gearbox 30 is in a planetary arrangement, thegearbox support 40 is coupled to the ring gear 38 so as to resist itsrotation relative to the static structure of the engine core 11.

The gearbox support 40 is defined between the point at which it connectsto the gearbox (e.g. to the planet carrier 34 in the present embodimentwith a star gearbox 30, or to the ring gear 38 in planetary embodiments)and a point at which it connects to the stationary supporting structure24. The gearbox support 40 may be formed by any number of separatecomponents providing a coupling between those two points. The gearboxsupport 40 connects to the gearbox 30 to the static gear or gearset—i.e. to the ring gear 38 of a planetary gearbox or the planetcarrier/planet gear set 34 of a star gearbox.

The gearbox support 40 has a degree of flexibility. Gearbox supporttorsional stiffness is a measure of the resistance of the support 40 toan applied torque, τ, as illustrated in FIG. 20. It may be described asresistance to twisting, or winding, of the support 40. The axis of themoment is parallel to the engine axis 9.

For a star gearbox 30, the torsional stiffness of the gearbox support 40is defined between a circle 902 passing through the centre of eachplanet gear 32 of the planetary gear set through the longitudinal axisof each pin 33) and the interface to the stationary support structure24, which is treated as fixed. The torsional load is applied at theplanet carrier 34, and reacted at the stationary support structure 24.

For a planetary gearbox 30, the torsional stiffness of the gearboxsupport 40 is defined between the pitch circle diameter (PCD) of thering gear 38, and the interface to the stationary support structure 24,which is treated as fixed. The torsional load is applied at the ringgear 38, and reacted at the stationary support structure 24.

The diagonal lines on the stationary support structure 24 are providedto indicate the connection to the support 40 being treated as rigid andnon-rotating.

For the example of a planetary gearbox 30, a torque, τ, is applied tothe teeth of the ring gear 38 and causes a rotational deformation, θ(e.g. twist) of the support 40. The angle through which a point on thePCD moves is θ, where θ is the angle measured in radians. An effectivelinear torsional stiffness can therefore be defined for the gearboxsupport 40 for a planetary gearbox 30 as described above using theradius r=PCD/2. Here, r is the radius of the ring gear 38 (i.e. half ofthe PCD of the ring gear).

In various embodiments the PCD of the ring gear 38, which may also bethought of as a diameter of the gearbox 30, may be greater than or equalto 0.55 in, and optionally greater than or equal to 0.57 m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm, the gearbox diameter may be greater thanor equal to 0.55 m, and may be equal to 0.61 m. In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the gearbox diameter may be greater than or equal to 0.75m, and may be equal to 0.87 m.

In various embodiments the diameter of the gearbox 30 may be in therange from 0.55 m to 1.2 m, and optionally in the range from 0.57 to 1.0m. In some embodiments, for example in embodiments in which the fandiameter is in the range from 240 to 280 cm the gearbox diameter may bein the range from 0.55 to 0.70 m, and optionally may be in the rangefrom 0.58 to 0.65 m (and optionally may be equal to 0.61 in). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the gearbox diameter may be in the rangefrom 0.75 to 1.0 m, and optionally may be in the range from 0.8 to 0.9 m(and optionally may be equal to 0.87 m).

Correspondingly, an effective linear torsional stiffness can thereforebe defined for the gearbox support 40 for a star gearbox 30 as describedabove using the radius r of the circle 902 passing through thelongitudinal axis of each pin 33 on the carrier 34. The diameter of thiscircle 902 may be described as a PCD of the planetary gear set, or a pinPCD, so providing r=PCD/2 as for the planetary gearbox example. Invarious embodiments the PCD of the planetary gear set (the pin PCD) maybe in the range from 0.38 to 0.65 m, for example being equal to 0.4 m or0.55 m.

In various embodiments, the torsional stiffness of the gearbox support40 is greater than or equal to 4.2×10⁷ Nm/rad, and optionally greaterthan or equal to 4.8×10⁷ Nm/rad. In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280 cmthe torsional stiffness of the gearbox support 40 may be greater than orequal to 4.2×10⁷ Nm/rad, and optionally may be greater than or equal to5×10⁷ Nm/rad (and optionally may be equal to 6.1×10⁷ Nm/rad). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the torsional stiffness of the gearboxsupport 40 may be greater than or equal to 7.0×10⁷ Nm/rad, andoptionally may be greater than or equal to 9×10⁷ Nm/rad (and optionallymay be equal to 1.8×10⁸ Nm/rad).

In various embodiments, the torsional stiffness of the gearbox support40 is in the range from 4.2×10⁷ to 1.0×10¹⁰ Nm/rad, and optionally inthe range from 4.8×10⁷ to 1.0×10⁹ Nm/rad. In some embodiments, forexample in embodiments in which the fan diameter is in the range from240 to 280 cm the torsional stiffness of the gearbox support 40 may bein the range from 4.2×10⁷ to 6.0×10⁸ Nm/rad, and optionally may be inthe range from 5×10⁷ to 7×10⁷ Nm/rad (and optionally may be equal to6.1×10⁷ Nm/rad). In some embodiments, for example in embodiments inwhich the fan diameter is in the range from 330 to 380 cm, the torsionalstiffness of the gearbox support 40 may be in the range from 7.0×10⁷ to1.0×10¹⁰ Nm/rad, and optionally may be in the range from 9×10⁷ to 4×10⁸Nm/rad (and optionally may be equal to 1.8×10⁸ Nm/rad).

In various embodiments, the effective linear torsional stiffness of thegearbox support 40 is greater than or equal to 7.1×10⁸ N/m, andoptionally greater than or equal to 8.4×10⁸ N/m. In some embodiments,for example in embodiments in which the fan diameter is in the rangefrom 240 to 280 cm the effective linear torsional stiffness of thegearbox support 40 may be greater than or equal to 7.1×10⁸ N/m, andoptionally may be greater than or equal to 8×10⁸ N/m (and optionally maybe equal to 9.2×10⁸ N/m). In some embodiments, for example inembodiments in which the fan diameter is in the range from 330 to 380cm, the effective linear torsional stiffness of the gearbox support 40may be greater than or equal to 9.0×10⁸ N/m, and optionally may begreater than or equal to 1.0×10⁹ N/m (and optionally may be equal to1.2×10⁹ N/m).

In various embodiments, the effective linear torsional stiffness of thegearbox support 40 is in the range from 7.1×10⁸ to 6.0×10¹⁰ N/m, andoptionally in the range from 8.4×10⁸ to 3.0×10¹⁰ N/m. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm the effective linear torsional stiffness ofthe gearbox support 40 may be in the range from 7.1×10⁸ to 5.0×10¹⁰ N/m,and optionally may be in the range from 8×10⁸ to 1×10⁹ N/m (andoptionally may be equal to 9.2×10⁸ N/m). In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the effective linear torsional stiffness of the gearboxsupport 40 may be in the range from 9.0×10⁸ to 6.0×10¹⁰ N/m, andoptionally may be in the range from 9.0×10⁸ or 1.0×10⁹ N/m to 2.0×10⁹N/m (and optionally may be equal to 1.2×10⁹ N/m).

One or more of gearbox support 40 geometry, materials, and connectiontype for the connection to the stationary support structure 24 may beselected or adjusted as appropriate to obtain the desired stiffness. Theskilled person would appreciate that the stiffness of the gearboxsupport 40 may be defined in a corresponding way for embodiments withdifferent epicyclic gearboxes.

The inventor has discovered that particular ratios of the parametersdefined above have significant impact on gearbox performance. Inparticular, one, some or all of the below conditions may apply to anyembodiment:

In various embodiments, the overall gear mesh stiffness of the gearbox30 is greater than or equal to 1.05×10⁹N/m, and optionally in the rangefrom 1.05×10⁹ to 8.00×10⁹ N/m. The gearbox diameter and/or the overallgear mesh stiffness of the gearbox 30 in such embodiments may fit withinany of the ranges specified above.

In various embodiments, a ring to sun mesh ratio of:

$\frac{\;\begin{matrix}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu} 32\mspace{14mu}{and}\mspace{14mu}{the}}\mspace{11mu}} \\{{ring}\mspace{14mu}{gear}\mspace{14mu} 38}\end{matrix}}{\mspace{14mu}\begin{matrix}{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu} 32\mspace{14mu}{and}\mspace{14mu}{the}} \\{{sun}\mspace{14mu}{gear}\mspace{14mu} 28}\end{matrix}}$is less than or equal to 1.28, and optionally less than or equal to1.235 or less than or equal 1.23. In alternative or additionalembodiments, the ring to sun mesh ratio may be greater than or equal to0.9, and optionally in the range from 0.9 to 1.3. or from 0.90 to 1.28.

In various embodiments, the ring to sun mesh ratio is in the range from9.00×10⁻¹ to 1.28×10⁰ (i.e. 0.900 to 1.28) and optionally from 0.95 to1.23. In some embodiments, for example in embodiments in which the fandiameter is in the range from 240 to 280 cm the ring to sun mesh ratiomay be in the range from 0.95 to 1.28, and optionally may be in therange from 0.95 to 1.23 (and optionally may be equal to 1.21). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the ring to sun mesh ratio may be in therange from 0.9 to 1.23 (and optionally may be equal to 1.15).

In various embodiments, a product of the components of the ring to sunmesh ratio, i.e. the gear mesh stiffness between the planet gears 32 andthe ring gear 38 multiplied by the gear mesh stiffness between theplanet gears 32 and the sun gear 28, may be calculated. The value ofthis product, in various embodiments, may be greater than or equal to4.7×10¹⁸ N²m⁻², and optionally less than 1.5×10¹⁹ N²m⁻², and optionallymay be greater than or equal to 5.1×10¹⁸ N²m⁻², and optionally less than1.3×10¹⁹ N²m⁻². In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 240 to 280 cm, the product valuemay be greater than or equal to 4.7×10¹⁸ N²m⁻², and optionally less than8.0×10¹⁸ N²m⁻². In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 330 to 380 cm, the product valuemay be greater than or equal to 6.0×10¹⁸ N²m⁻², and optionally less than1.5×10¹⁹ N²m⁻².

In various embodiments, a carrier to sun mesh ratio of:

$\frac{\begin{matrix}{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{11mu}{the}} \\{{planet}\mspace{14mu}{carrier}\mspace{14mu} 34}\end{matrix}}{\mspace{14mu}\begin{matrix}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu} 32\mspace{14mu}{and}\mspace{14mu}{the}}\mspace{14mu}} \\{{sun}\mspace{14mu}{gear}\mspace{14mu} 28}\end{matrix}}$is greater than or equal to 2.60×10⁻¹.

In various embodiments, the carrier to sun mesh ratio may be greaterthan or equal to 2.60×10⁻¹, and optionally greater than or equal to4.5×10⁰, and further optionally greater than or equal to 5.1. In someembodiments, for example in embodiments in which the fan diameter is inthe range from 240 to 280 cm the carrier to sun mesh ratio may begreater than or equal to 0.6, and optionally may be greater than orequal to 2 or 5 (and optionally may be equal to 5.82). In someembodiments, for example in embodiments in which the fan diameter is inthe range from 330 to 380 cm, the carrier to sun mesh ratio may begreater than or equal to 0.94, and optionally greater than or equal to 5(and optionally may be equal to 9.47).

In various embodiments, the carrier to sun mesh ratio is in the rangefrom 2.60×10⁻¹ to 1.10×10³, and optionally from 4.5×10⁰ or 5.1×10⁰ to9.5×10¹ (i.e. from 4.5 or 5.1 to 95). In some embodiments, for examplein embodiments in which the fan diameter is in the range from 240 to 280cm the carrier to sun mesh ratio may be in the range from 0.6 to 58, andoptionally may be in the range from 2 or 5 to 10 (and optionally may beequal to 5.82). In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 330 to 380 cm, the carrier to sunmesh ratio may be in the range from 0.94 to 95, and optionally in therange from 5 to 16 (and optionally may be equal to 9.47).

In various embodiments, a product of the components of the carrier tosun mesh ratio, i.e. the effective linear torsional stiffness of theplanet carrier 34 multiplied by the gear mesh stiffness between theplanet gears 32 and the sun gear 28, may be calculated. The value ofthis product, in various embodiments, may be greater than or equal to5.0×10¹⁸N²m⁻², and optionally less than 2.0×10²² N²m⁻², optionally maybe greater than or equal to 1.8×10¹⁹ N²m⁻², and optionally less than1.0×10²¹ N²m⁻². In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 240 to 280 cm, the product valuemay be greater than or equal to 1.8×10¹⁹ N²m⁻², and optionally less than8.0×10²⁰ N²m⁻². In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 330 to 380 cm, the product valuemay be greater than or equal to 5.0×10¹⁹ N²m⁻², and optionally less than8.0×10²¹ N²m⁻².

In various embodiments, a carrier to ring mesh ratio of:

$\frac{\begin{matrix}{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{11mu}{the}} \\{{planet}\mspace{14mu}{carrier}\mspace{14mu} 34}\end{matrix}}{\mspace{14mu}\begin{matrix}{{{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{planet}\mspace{14mu}{gears}\mspace{14mu} 32\mspace{14mu}{and}\mspace{14mu}{the}}\mspace{14mu}} \\{{ring}\mspace{14mu}{gear}\mspace{14mu} 38}\end{matrix}}$is greater than or equal to 0.2.

In various embodiments, the carrier to ring mesh ratio may be greaterthan or equal to 2.00×10⁻¹ (i.e. 0.200), and optionally may be greaterthan or equal to 3.8×10⁰ (i.e. 3.8). In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280 cmthe carrier to ring mesh ratio may be greater than or equal to 3.8, andoptionally greater than or equal to 3.9 (and optionally may be equal to4.79). In some embodiments, for example in embodiments in which the fandiameter is in the range from 330 to 380 cm, the carrier to ring meshratio may be greater than or equal to 4.0, and optionally may be greaterthan or equal to 5 (and optionally may be equal to 8.24).

In various embodiments, the carrier to ring mesh ratio is in the rangefrom 2.00×10⁻¹ to 9.00×10², and optionally from 3.8×10⁰ to 9.0×10¹ (i.e.from 3.8 to 90). In some embodiments, for example in embodiments inwhich the fan diameter is in the range from 240 to 280 cm, the carrierto ring mesh ratio may be in the range from 3.8 to 90, and optionallymay be in the range from 3.9 to 7.0 (and optionally may be equal to4.79). In some embodiments, for example in embodiments in which the fandiameter is in the range from 330 to 380 cm, the carrier to ring meshratio may be in the range from 4.0 to 5.0×10², and optionally in therange froth 5 to 20 (and optionally may be equal to 8.24).

In various embodiments, a product of the components of the carrier toring mesh ratio, i.e. the effective linear torsional stiffness of theplanet carrier 34 multiplied by the gear mesh stiffness between theplanet gears 32 and the ring gear 38, may be calculated. The value ofthis product, in various embodiments, may be greater than or equal to5.0×10¹⁸N²m⁻², and optionally less than 2.6×10²² N²m⁻², and optionallymay be greater than or equal to 2.2×10¹⁹ N²m⁻², and optionally less than2.6×10²¹ N²m⁻². In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 240 to 280 cm, the product valuemay be greater than or equal to 2.2×10¹⁹N²m⁻², and optionally less than2.6×10²¹ N²m⁻². In some embodiments, for example in embodiments in whichthe fan diameter is in the range from 330 to 380 cm, the product valuemay be greater than or equal to 2.5×10¹⁹ N²m⁻², and optionally less than6.0×10²¹ N²m⁻².

In various embodiments, a gear mesh to transmission stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu} 30}{{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{transmission}}$is less than or equal to 11, and optionally less than or equal to 4.6.

In various embodiments, the gear mesh to transmission stiffness ratio isless than or equal to 2.7, and optionally less than or equal to 2.70.

In various embodiments, the gear mesh to transmission stiffness ratio isin the range from 3.4×10⁻¹ to 1.1×10¹ (i.e. from 0.34 to 11), andoptionally in the range from 0.90 to 4.6. In some embodiments, forexample in embodiments in which the fan diameter is in the range from240 to 280 cm the gear mesh to transmission stiffness ratio may be inthe range from 1.4 to 2.7, and optionally may be in the range from 2.0to 2.6 (and optionally may be equal to 2.45). In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the gear mesh to transmission stiffness ratio may be inthe range from 0.50 to 4.6, and optionally in the range from 1.2 to 2.3(and optionally may be equal to 1.99).

In various embodiments, a gear mesh and transmission stiffness productof:overall gear mesh stiffness of the gearbox 30×effective linear torsionalstiffness of the transmissionis greater than or equal to 1.6×10¹⁷ N²m⁻², and optionally greater thanor equal to 3.2×10¹⁷ N²m⁻². In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280cm, the gear mesh and transmission stiffness product may be greater thanor equal to 4.2×10¹⁷ N²m⁻². In some embodiments, for example inembodiments in which the fan diameter is in the range from 330 to 380cm, the gear mesh and transmission stiffness product may be greater thanor equal to 5.8×10¹⁷ N²m⁻².

In various embodiments, the gear mesh and transmission stiffness productis in the range from 1.6×10¹⁷ to 2.9×10¹⁹ N²m⁻², and optionally in therange from 3.2×10¹⁷ to 1.5×10¹⁹ N²m⁻². In some embodiments, for examplein embodiments in which the fan diameter is in the range from 240 to 280cm, the gear mesh and transmission stiffness product may be in the rangefrom 4.2×10¹⁷ to 1.5×10¹⁹ N²m⁻². In some embodiments, for example inembodiments in which the fan diameter is in the range from 330 to 380cm, the gear mesh and transmission stiffness product may be in the rangefrom 5.8×10¹⁷ to 2.9×10¹⁹ N²m⁻².

In various embodiments, a gear mesh to fan shaft stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu} 30}{\begin{matrix}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}} \\{{fan}\mspace{14mu}{shaft}\mspace{14mu} 36}\end{matrix}}$is less than or equal to 1.6, and optionally greater than 0.3.

In various embodiments, the gear mesh to fan shaft stiffness ratio maybe less than or equal to 1.6×10° (i.e. 1.6), and optionally may be lessthan or equal to 0.85. In some embodiments, the gear mesh to fan shaftstiffness ratio may be less than or equal to 0.80, and optionally may beless than or equal to 0.79 or 0.75.

In various embodiments, the gear mesh to fan shaft stiffness ratio maybe in the range from 3.0×10⁻¹ (i.e. 0.30) to 1.6×10⁰) (i.e. 1.6), andoptionally may be in the range from 0.4 to 0.85.

In some embodiments, the gear mesh to fan shaft stiffness ratio may bein the range from 0.45 to 0.80, and optionally may be in the range from0.50 to 0.75. For example, the gear mesh to fan shaft stiffness ratiomay be at least substantially equal to 0.78, for example being 0.782 or0.778.

In various embodiments, a product of the components of the gear mesh tofan shaft stiffness ratio, i.e. the overall gear mesh stiffness of thegearbox 30 multiplied by the effective linear torsional stiffness of thefan shaft 36, may be calculated. The value of this product, in variousembodiments, may be greater than or equal to 1.3×10¹⁸N²m⁻², andoptionally less than 5.0×10¹⁹ N²m⁻², and optionally may be greater thanor equal to 1.4×10¹⁸ N²m⁻², and optionally, less than 3.0×10¹⁹ N²m⁻². Insome embodiments, for example in embodiments in which the fan diameteris in the range from 240 to 280 cm, the product value may be greaterthan or equal to 1.5×10¹⁸ N²m⁻², and optionally less than 8.5×10¹⁸N²m⁻². In some embodiments, for example in embodiments in which the fandiameter is in the range from 330 to 380 cm, the product value may begreater than or equal to 1.7×10¹⁸ N²m⁻², and optionally less than5.0×10¹⁹ N²m⁻².

In various embodiments, a gear mesh to core shaft stiffness ratio of:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu} 30}{\begin{matrix}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}} \\{{core}\mspace{14mu}{shaft}\mspace{14mu} 26}\end{matrix}}$is greater than or equal to 0.2, and optionally smaller than or equal to90 or smaller than or equal to 29, and further optionally smaller thanor equal to 2.9.

In various embodiments, the gear mesh to core shaft stiffness ratio isless than or equal to 2.9×10¹, and optionally less than or equal to9.0×10¹. In some embodiments, the gear mesh to core shaft stiffnessratio may be less than or equal to 2.4, and optionally may be less thanor equal to 2.40. In some embodiments the gear mesh to core shaftstiffness ratio may be less than or equal to 2.9×10¹, and optionally maybe less than or equal to 2.50, or to 2.38.

In various embodiments, the gear mesh to core shaft stiffness ratio isin the range from 2.0×10⁻¹ to 2.9×10¹, and optionally in the range from9.0×10⁻¹ to 9.0×10¹. In some embodiments, the gear mesh to core shaftstiffness ratio may be in the range from 2.0×10⁻¹ to 2.4, and optionallymay be in the range from 2.25 to 2.4 (and optionally may be equal to2.40). In some embodiments, the gear mesh to core shaft stiffness ratiomay be in the range from 2.4 to 2.9×10¹, and optionally may be in therange from 2.10 to 2.50 (and optionally may be equal to 2.38).

In various embodiments, a gear mesh and core shaft stiffness product of:overall gear mesh stiffness of the gearbox 30×effective linear torsionalstiffness of the core shaft 26is in the range from 1.0×10¹⁷ to 3.0×10¹⁹ N²m⁻², and optionally in therange from 4.5×10¹⁷ to 9.0×10¹⁸ N²m⁻². In some embodiments, for examplein embodiments in which the fan diameter is in the range from 240 to 280cm, the gear mesh and core shaft stiffness product may be in the rangefrom 4.5×10¹⁷ to 8.0×10¹⁸ N²m⁻². In some embodiments, for example inembodiments in which the fan diameter is in the range from 330 to 380cm, the gear mesh and core shaft stiffness product may be in the rangefrom 8.0×10¹⁷ to 3.0×10¹⁹ N²m⁻².

In various embodiments, a gear mesh to gearbox support stiffness ratioof:

$\frac{{the}\mspace{14mu}{overall}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu} 30}{\begin{matrix}{{the}\mspace{14mu}{effective}\mspace{14mu}{linear}\mspace{14mu}{torsional}} \\{{stiffness}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{gearbox}\mspace{14mu}{support}\mspace{14mu} 40}\end{matrix}}$is in the range from 6.5×10⁻² to 2.6×10¹, and optionally in the rangefrom 1.0 to 1.6.

In various embodiments, the gear mesh to gearbox support stiffness ratiois in the range from 6.5×10⁻² to 2.6×10¹, and optionally in the rangefrom 2.6×10⁻¹ to 8.0. In some embodiments, the gear mesh to gearboxsupport stiffness ratio may be in the range from 6.5×10⁻² to 1.1, orfrom 6.5×10⁻² to 1.4, and optionally may be in the range from 1.20 to1.32 (and optionally may be equal to 1.29). In some embodiments, thegear mesh to gearbox support stiffness ratio may be in the range from1.1 to 2.6×10¹, and optionally may be in the range from 1.34 to 1.60(and optionally may be equal to 1.37).

In various embodiments, a gear mesh and gearbox support stiffnessproduct of:overall gear mesh stiffness of the gearbox 30×the effective lineartorsional stiffness of the gearbox support 40is greater than or equal to 2.0×10¹⁷ N²m⁻², and optionally greater thanor equal to 9.0×10¹⁷ N²m⁻². In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280cm, the gear mesh and core shaft stiffness product may be greater thanor equal to 5.0×10¹⁷ N²m⁻². In some embodiments, for example inembodiments in which the fan diameter is in the range from 330 to 380cm, the gear mesh and core shaft stiffness product may be greater thanor equal to 1.0×10¹⁸ N²m⁻².

In various embodiments, the gear mesh and gearbox support stiffnessproduct is in the range from 2.0×10¹⁷ to 4.1×10¹⁹ N²m⁻², and optionallyin the range from 9.0×10¹⁷ to 7.1×10¹⁹ N²m⁻². In some embodiments, forexample in embodiments in which the fan diameter is in the range from240 to 280 cm, the gear mesh and core shaft stiffness product may be inthe range from 5.0×10¹⁷ to 1.0×10¹⁹ N²m⁻². In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the gear mesh and core shaft stiffness product may be inthe range from 1.0×10¹⁸ to 4.1×10¹⁹ N²m⁻².

FIG. 19 illustrates how the stiffnesses defined herein may be measured.FIG. 19 shows a plot of the displacement δ resulting from theapplication of a load L (e.g. a force, moment or torque) applied to acomponent for which the stiffness is being measured. At levels of loadfrom zero to L_(P) there is a non-linear region in which displacement iscaused by motion of the component (or relative motion of separate partsof the component) as it is loaded, rather than deformation of thecomponent; for example moving within clearance between parts. At levelsof load above L_(Q) the elastic limit of the component has been exceededand the applied load no longer causes elastic deformation—plasticdeformation or failure of the component may occur instead. Betweenpoints P and Q the applied load and resulting displacement have a linearrelationship. The stiffnesses defined herein may be determined bymeasuring the gradient of the linear region between points P and Q (withthe stiffness being the inverse of that gradient). The gradient may befound for as large a region of the linear region as possible to increasethe accuracy of the measurement by providing a larger displacement tomeasure. For example, the gradient may be found by applying a load equalto or just greater than L_(P) and equal to or just less than L_(Q).Values for L_(P) and L_(Q) may be estimated prior to testing based onmaterials characteristics so as to apply suitable loads. Although thedisplacement is referred to as δ in this description, the skilled personwould appreciate that equivalent principles would apply to a linear orangular displacement.

The stiffnesses defined herein, unless otherwise stated, are for thecorresponding component(s) when the engine is off (i.e. at zero speed/onthe bench). The stiffnesses generally do not vary significantly over theoperating range of the engine; the stiffness at cruise conditions of theaircraft to which the engine is used (those cruise conditions being asdefined elsewhere herein) may therefore be the same as for when theengine is not in use. However, where the stiffness varies over theoperating range of the engine, the stiffnesses defined herein are to beunderstood as being values for when the engine is at room temperatureand unmoving.

The present disclosure also relates to methods 1300 of operating a gasturbine engine 10 on an aircraft. The methods 1300 are illustrated inFIG. 21.

The method 1300 comprises starting up and operating 1302 the engine 10(e.g. taxiing on a runway, take-off, and climb of the aircraft, assuitable) to reach cruise conditions. Once cruise conditions have beenreached, the method 1300 then comprises operating 1304 the gas turbineengine 10, which may be as described in one or more embodimentselsewhere herein, to provide propulsion under cruise conditions.

The gas turbine engine 10 is such that, and/or is operated such that,any or all of the parameters or ratios defined herein are within thespecified ranges.

The torque on the core shaft 26 may be referred to as the input torque,as this is the torque which is input to the gearbox 30. The torquesupplied by the turbine 19 to the core shaft the torque on the coreshaft) at cruise conditions may be greater than or equal to 10,000 Nm,and optionally greater than or equal to 11,000 Nm. In some embodiments,for example in embodiments in which the fan diameter is in the rangefrom 240 to 280 cm, the torque on the core shaft 26 at cruise conditionsmay be greater than or equal to 10,000 or 11,000 Nm (and optionally maybe equal to 12,760 Nm). In some embodiments, for example in embodimentsin which the fan diameter is in the range from 330 to 380 cm, the torqueon the core shaft 26 at cruise conditions may be greater than or equalto 25,000 Nm, and optionally greater than or equal to 30,000 Nm (andoptionally may be equal to 34,000 Nm).

The torque on the core shaft at cruise conditions may be in the rangefrom 10,000 to 50,000 Nm, and optionally from 11,000 to 45,000 Nm. Insome embodiments, for example in embodiments in which the fan diameteris in the range from 240 to 280 cm, the torque on the core shaft 26 atcruise conditions may be in the range from 10,000 to 15,000 Nm, andoptionally from 11,000 to 14,000 Nm (and optionally may be equal to12,760 Nm). In some embodiments, for example in embodiments in which thefan diameter is in the range from 330 to 380 cm, the torque on the coreshaft 26 at cruise conditions may be in the range from 25,000 Nm to50,000 Nm, and optionally from 30,000 to 40,000 Nm (and optionally maybe equal to 34,000 Nm).

Under maximum take-off (MTO) conditions, the torque on the core shaft 26may be greater than or equal to 28,000 Nm, and optionally greater thanor equal to 30,000 Nm. In some embodiments, for example in embodimentsin which the fan diameter is in the range from 240 to 280 cm, the torqueon the core shaft 26 under MTO conditions may be greater than or equalto 28,000, and optionally greater than or equal to 35,000 Nm (andoptionally may be equal to 36,300 Nm). In some embodiments, for examplein embodiments in which the fan diameter is in the range from 330 to 380cm, the torque on the core shaft 26 under MTO conditions may greaterthan or equal to 70,000 Nm, and optionally greater than or equal to80,000 or 82,000 Nm (and optionally may be equal to 87,000 Nm).

Under maximum take-off (MTO) conditions, the torque on the core shaft 26may be in the range from 28,000 Nm to 135,000 Nm, and optionally in therange from 30,000 to 110,000 Nm. In some embodiments, for example inembodiments in which the fan diameter is in the range from 240 to 280cm, the torque on the core shaft 26 under MTO conditions may be in therange from 28,000 to 50,000 Nm, and optionally from 35,000 to 38,000 Nm(and optionally may be equal to 36,300 Nm). In some embodiments, forexample in embodiments in which the fan diameter is in the range from330 to 380 cm, the torque on the core shaft 26 under MTO conditions maybe in the range from 70,000 Nm to 135,000 Nm, and optionally from 80,000to 90,000 Nm or 82,000 to 92,000 Nm (and optionally may be equal to87,000 Nm).

Torque has units of [force]×[distance] and may be expressed in units ofNewton metres (N·m), and is defined in the usual way as would beunderstood by the skilled person.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and sub-combinations of one or morefeatures described herein.

The invention claimed is:
 1. A gas turbine engine for an aircraftcomprising: an engine core comprising a turbine, a compressor, and acore shaft connecting the turbine to the compressor; a fan locatedupstream of the engine core, the fan comprising a plurality of fanblades; and a gearbox arranged to receive an input from the core shaftand to output drive to the fan so as to drive the fan at a lowerrotational speed than the core shaft, the gearbox being an epicyclicgearbox comprising: a sun gear, a plurality of planet gears, a ringgear, and a planet carrier on which the plurality of planet gears aremounted, the gearbox having a first gear mesh stiffness between theplurality of planet gears and the ring gear and a second gear meshstiffness between the plurality of planet gears and the sun gear, andwherein: a ring to sun mesh ratio of: $\frac{\begin{matrix}{{{the}\mspace{14mu}{first}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{plurality}\mspace{14mu}{of}}\mspace{11mu}} \\{{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{ring}\mspace{14mu}{gear}}\end{matrix}}{\begin{matrix}{{the}\mspace{14mu}{second}\mspace{14mu}{gear}\mspace{14mu}{mesh}\mspace{14mu}{stiffness}\mspace{14mu}{between}\mspace{14mu}{the}\mspace{14mu}{plurality}\mspace{14mu}{of}} \\{{planet}\mspace{14mu}{gears}\mspace{14mu}{and}\mspace{14mu}{the}\mspace{14mu}{sun}\mspace{14mu}{gear}}\end{matrix}}$ is in the range from 0.90 to 1.28.
 2. The gas turbineengine of claim 1 wherein the ring to sun mesh ratio is in the rangefrom 0.95 to 1.23.
 3. The gas turbine engine of claim 1 wherein thegearbox has a gearbox diameter defined as the pitch circle diameter ofthe ring gear, and the gearbox diameter is in the range from 0.55 to 1.2m.
 4. The gas turbine engine of claim 1 wherein the first gear meshstiffness between the plurality of planet gears and the ring gear isgreater than or equal to 1.4×10⁹ N/m and less than or equal to 2.0×10¹⁰N/m.
 5. The gas turbine engine of claim 1 wherein the second gear meshstiffness between the plurality of planet gears and the sun gear isgreater than or equal to 1.20×10⁹ N/m and less than or equal to1.60×10¹⁰ N/m.
 6. The gas turbine engine of claim 1, wherein either: (i)the fan has a fan diameter in the range from 240 to 280 cm, and the ringto sun mesh ratio is in the range from 0.95 to 1.28; or (ii) the fan hasa fan diameter in the range from 330 to 380 cm, and the ring to sun meshratio is in the range from 0.90 to 1.23.
 7. The gas turbine engine ofclaim 1 wherein the first gear mesh stiffness between the plurality ofplanet gears and the ring gear multiplied by the second gear meshstiffness between the plurality of planet gears and the sun gear isgreater than or equal to 4.7×10¹⁸ N²m⁻² and less than 1.5×10¹⁹ N²m⁻². 8.The gas turbine engine of claim 1 wherein a torsional stiffness of theplanet carrier is greater than 1.60×10⁸ Nm/rad and less than 1.0×10¹¹Nm/rad.
 9. The gas turbine engine according to claim 1, wherein theplanet carrier comprises a forward plate and a rearward plate and pinsextending therebetween, each pin being arranged to have a planet gearmounted on each pin between the forward plate and rearward plate. 10.The gas turbine engine according to claim 1, wherein the gearboxcomprises an odd number of planet gears.
 11. The gas turbine engineaccording to claim 1, wherein: the turbine is a first turbine, thecompressor is a first compressor, and the core shaft is a first coreshaft; the engine core further comprises a second turbine, a secondcompressor, and a second core shaft connecting the second turbine to thesecond compressor; and the second turbine, a second compressor, and asecond core shaft are arranged to rotate at a higher rotational speedthan the first core shaft.
 12. The gas turbine engine according to claim1, wherein the fan has a fan diameter greater than 240 cm and less thanor equal to 380 cm.
 13. The gas turbine engine of claim 1, wherein thegas turbine engine comprises a fan shaft extending between the gearboxand the fan, and a gearbox support arranged to mount the gearbox withinthe gas turbine engine, the fan shaft, core shaft, the gearbox, and thegearbox support together forming a transmission, and wherein theeffective linear torsional stiffness of the transmission is greater thanor equal to 1.60×10⁸ N/m and less than or equal to 8.2×10⁸ N/m.